From BiblePortal Wikipedia

Hastings' Dictionary of the Bible [1]

Geshem (  Nehemiah 2:18;   Nehemiah 6:1-2; in   Nehemiah 6:6 the form Gashmu occurs). An Arabian who is named, along with Sanballat the Horonite and Tobiah the Ammonite, as an opponent of Nehemiah during the rebuilding of the walls of Jerusalem (  Nehemiah 2:16;   Nehemiah 6:1 ff.). He may have belonged to an Arab community which, as we learn from the monuments, was settled by Sargon in Samaria c [Note: circa, about.] . b.c. 715 this would explain his close connexion with the Samaritans; or he may have been the chief of an Arab tribe dwelling in the S. of Judah, in which case his presence would point to a coalition of all the neighbouring peoples against Jerusalem.

Fausset's Bible Dictionary [2]

An Arab who, with Sanballat of Horonaim, and Tobiah the servant, the Ammonite, opposed Nehemiah in repairing Jerusalem. ( Nehemiah 2:19;  Nehemiah 6:1, etc.) Frustrated in this as well as in the plot against Nehemiah's life. It was for the interest of the wandering marauders of the frontier of Palestine to prevent its restoration as a kingdom.

Morrish Bible Dictionary [3]

An Arabian, who with Sanballat and Tobiah sought to hinder the rebuilding of Jerusalem.  Nehemiah 2:19;  Nehemiah 6:1,2 . Called also GASHMU in  Nehemiah 6:6 .

Holman Bible Dictionary [4]

 Nehemiah 2:19 Nehemiah 6:1-19 Nehemiah 6:6

Easton's Bible Dictionary [5]

 Nehemiah 2:19 6:1,2

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [6]

(Heb. id. גֶּשֶׁם a Shower, if Heb.; F Ü rst, Firmness; but more prob. the Arabic Jasim or Jahum, a historical name in Arabia Proper; Sept. Γηρσώμ , Vulg. Gesam ), once ( Nehemiah 6:6) in the prolonged form GASH'MU (He.). Gashmu' , גִּשְׁמוּ ), as Arabian ( Nehemiah 2:19;  Nehemiah 6:1), and one of the enemies of the Jews on the returns from the exile, especially in the plots against the life of Nehemiah ( Nehemiah 6:2). B.C. 446. Geshem, we may conclude, was an inhabitant of Arabia Petraea, or of the Arabian Desert, and probably the chief of a tribe which, like most of the tribes on the osastern frontier of Palestine, was, in the time of the captivity and the subsequent period, allied with the Persians, or with any peoples threatening the Jewish nation. Geshems, like Sanballat and Tobiab, seems to have been one of the "governors beyond the river," to whom Nehemiah came, and whose mission "grieved them exceedingly, that there was come a man to seek the welfare of the children of Israel" ( Nehemiah 2:10); for the wsardering inhabitants of the frontier doubtless availed themnselves largely, in their predatory excursions, of the distracted state of Palestine, and dreaded the re-establishment of the kingdom; and the Arabians, Ammonites, and Ashdodites are recorded as having "conspired to fight against Jerusalem, and to hinder" its repairing. (See Nehemiah).

International Standard Bible Encyclopedia [7]

gē´shem ( גּשׁם , geshem , גּשׁמוּ , gashmū  ; Γῆσαμ , Gḗsam , "rain storm"): An Arabian, probably chief of an Arabian tribe that had either settled in Southern Palestine during the exile in Babylon, or had been settled in or near Samaria by Sargon (  Nehemiah 2:19;  Nehemiah 6:1 ,  Nehemiah 6:2 ,  Nehemiah 6:6 ). He was a confederate of Sanballat and Tobiah, and strenuously opposed the building of the wall under Nehemiah. He with the others mocked at the first efforts to build the wall, and afterward repeatedly sought to entice Nehemiah to the plains of Ono. The name also occurs in the form Gashmū , perhaps an Assyrian form of the same name Geshem.

Kitto's Popular Cyclopedia of Biblial Literature [8]

Ge´shem (rain). One of the enemies of the Jews under Nehemiah . He was probably a Samaritan, although on some account or other designated an Arabian , and seems to have been a subaltern officer at Jerusalem. He opposed the designs of the Jewish governor, talking of them as seditious, and turning them into ridicule. Eventually he took part in the plots of Tobiah against the life of Nehemiah , about B.C. 445.