From BiblePortal Wikipedia

Watson's Biblical & Theological Dictionary [1]

a name given to the collective political body of Israelites. Some derive the word from ישר , just or righteous, and so make it to signify a righteous people. Montanus renders it rectitudo, and so does the Samaritan version. But it seems a considerable objection against this sense, that Israel is called Jeshurun at the very time that they are upbraided with their sins and their rebellion: "Jeshurun waxed fat, and kicked," &c,   Deuteronomy 32:15 . It is replied, Jeshurun is the diminutive of ישר , (for nomen auctum in fine est nomen diminutivum, ) and so imports, that though, in general and on the whole, they were a righteous people, yet they were not without great faults. Perhaps Cocceius has given as probable an interpretation as any. He derives the word from שור , which signifies go see, behold, or discover; from whence, in the future tense, plural, comes ישרו , which, with the addition of nun paragogicum, makes Jeshurun; that is, "the people who had the vision of God." This makes the name of Jeshurun to be properly applied to Israel, not only when Moses is called their king, but when they are upbraided with their rebellion against God; since the peculiar manifestation which God had made of himself to them was a great aggravation of their ingratitude and rebellion.

Hastings' Dictionary of the Bible [2]

JESHURUN . A poetic or a pet-name for Israel which occurs four times in the OT (  Deuteronomy 32:15;   Deuteronomy 33:5;   Deuteronomy 33:26 ,   Isaiah 44:2 ). It is found in the later writings, and represents a patriotic feeling that Israel was = yashar-Ei , ‘the upright of God.’ If this be so, then we may accept the rendering of Jeshurun as the ‘righteous little people.’ In Balaam’s elegy,’ Let me die the death of the righteous’ seems to refer to the Israel of the preceding clause, and in   Psalms 83:1 the thought which underlies Jeshurun appears, if we adopt the tempting reading: ‘Truly God is good to the upright.’

W. F. Cobb.

Smith's Bible Dictionary [3]

Jesh'urun. (Supremely Happy). Jeshurun and, once by mistake in Authorized Version, as Jesurun ,  Isaiah 44:2, a symbolical name for Israel in  Deuteronomy 32:15;  Deuteronomy 33:5;  Deuteronomy 23:26;  Isaiah 44:2. It is most probably derived from a root, signifying "to be blessed." With the intensive termination, Jeshurun would then denote Israel, as supremely happy or prosperous, and to this signification the context in  Deuteronomy 32:15 points.

Fausset's Bible Dictionary [4]

("the righteous (from Yaashaar ) people"): Israel's ideal character; his high calling ( Deuteronomy 32:15;  Deuteronomy 33:5;  Deuteronomy 33:26;  Isaiah 44:2; compare  Numbers 23:21). (See Jasher , Book Of ) The idea of blessedness and prosperity enters into the word; the Arabic and the Hebrew: Ashar , "happy," being related to Yaashaar . There is a play on similar sounds which the Hebrew writers delight in, in Jeshurun and the diminutive of Israel, Israelun.

People's Dictionary of the Bible [5]

Jeshurun ( Jĕsh'U-Rŭn or Je-Shû'Run ), Happy, and once in A. V. Jesurun,  Isaiah 44:2, a symbolical name for Israel in  Deuteronomy 32:15;  Deuteronomy 33:5;  Deuteronomy 33:26;  Isaiah 44:2. It is most probably derived from a root signifying "to be blessed."

American Tract Society Bible Dictionary [6]

A poetical name of Israel, probably derived from a root meaning to be upright, and applied to the people of God as the objects of his justifying love, which does not "behold iniquity in Jacob,"  Deuteronomy 32:5   33:5,26   Isaiah 44:2 .

Holman Bible Dictionary [7]

 Deuteronomy 32:15 Deuteronomy 33:5 33:26 Isaiah 44:2 Ecclesiastes 37:25

Easton's Bible Dictionary [8]

 Deuteronomy 32:15 33:5,26 Isaiah 44:2

International Standard Bible Encyclopedia [9]

je - shū ´- run , jesh ´- u - run ( ישׁרוּן , yeshurūn , "upright one,"   Deuteronomy 32:15;  Deuteronomy 33:5 ,  Deuteronomy 33:26;  Isaiah 44:2 ): Septuagint translates it "the beloved one" ἠγαπημένος , ēgapēménos , the perfect participle passive of agapáō ), and in  Isaiah 44:2 adds "Israel"; Vulgate (Jerome's Latin Bible, 390-405 A.D.) has dilectus in   Deuteronomy 32:15 , elsewhere rectissimus  ; Aquila, Symmachus, Theodotion have "upright." For the form, Duhm compares זבלוּן , zebhulūn , Zebulun. (1) The name used to be explained as a diminutive form, a pet name, and some, e.g. Cornill, Schultz ( Old Testament Theology , English translation, II, 29, note 12) still explain it so, "the righteous little people." But there is no evidence that the ending - ūn had a diminutive force. (2) Most moderns take it as a poetical or ideal title of Israel, derived from ישׁר , yāshār , "upright"; it is held to contain a tacit reference to the word Israel (ישׁראל , yisrā'ēl ), of which the first three consonants are almost the same as those of "Jeshurun"; in  Numbers 23:10 the term "the righteous ones" ( ישׁרים , yeshārı̄m ) is supposed to contain a similar reference. Most commentators compare also "the Book of Jashar," and it has been held that "Jashar" is similarly a name by which Israel is called. See Jashar .

Following Bacher ( Zatw , 1885,161 ff), commentators hold that in Isaiah this new name, a coinage due to the author of Second Isaiah and adopted in Deuteronomy, stands in contrast to Jacob, "the supplanter," as his name was explained by the Hebrews (compare  Hosea 12:2-4 ). Israel is here given a new name, "the upright, pious one," and with the new name goes new chance in life, to live up to its meaning. Driver (Deuteronomy, 361) says that in  Deuteronomy 32:15 "where the context is of declension from its ideal (it is) applied reproachfully. 'Nomen Recti pro Israele ponens, ironice eos perstringit qui a rectitudine defecerant' (Calv.). Elsewhere it is used as a title of honor." the King James Version has "Jesurun" in   Isaiah 44:2 .

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [10]

(Heb. Yeshurun', יְשֻׁרוּן ), a poetical appellation of the people of Israel, used in token of affection and tenderness, occurring four times ( Deuteronomy 32:15, Sept. Ι᾿Ακώβ , Vulg. Dilectus;  Deuteronomy 33:5;  Deuteronomy 33:26, and  Isaiah 44:2 [A. Vers. in this latter passage "Jesurun"]; Sept. Ἠγαπημένος , Vulgate Rectissimus ) . The term is (according to Mercer in Pagnini, Thes. 1, p. 1105; Mich. in Suppl., and others) a diminutive (after the form of Zebulun, Jeduthun, etc.) from יָשׁוּר i.q. יָשָׁר (compare שָׁלוּם and שָׁלֵם ), q.d. Rectulus, a rightling," i.e. the dear upright people. Aquila, Symmachus, and Theodotion have in Isaiah Εὐθύς , elsewhere Εύθύτατος ; Kimchi says, "Israel is so called as being Just among the nations;" so also Aben-Ezra and Saadias (in the Pent. ) interpret. Others, as Grotius, understand the word as a diminutive from "Israel" itself, and so apparently the Chald., Syriac, and Saadias (in Isaiah), but against the analogy of derivation. Ilgen ( D ) E Imnbre Lapideo, p. 25, and in Paulus, Memoreabil. 6, p. 157) gives a far fetched derivation from the Arabic, and other fanciful explanations may be seen in Jo. Olpius's Diss. De ישרון (praeside Theod. Hasaeo, Breme, 1730). The passages where it is employed seem to express the idea that in the character of Righteous Jehovah recognized his people in consideration of their covenant relation to him, whereby, while they observed the terms of that covenant, they stood legally justified before him and clean in his sight. It is in this sense that the pious kings are said to have done הִיָּשָׁר , "that which was right" in the eyes of Jehovah, i.e. what God approved ( 1 Kings 11:34, etc.).

Kitto's Popular Cyclopedia of Biblial Literature [11]

Jesh´urun, a name poetically applied to Israel in;;; . It has been very variously understood, but it is generally agreed to be a poetical diminutive expressive of affection. It is derived from a word signifying to be straight, right, upright, righteous. In this character, as entirely upright (for the termination is intensitive), Jehovah recognizes his people in consideration of their covenant relation to him, whereby, while they observed the terms of that covenant, they stood legally righteous before him and clean in his sight. It is in this sense that the ancient kings are said to have done 'that which was right' in the eyes of Jehovah.