From BiblePortal Wikipedia

Fausset's Bible Dictionary [1]

i.e. "chief cupbearer" (2 Kings 18-19; Isaiah 36-37). Sent by Sennacherib with Tartan who probably had chief command (first in  2 Kings 18:17;  Isaiah 20:1) of an army to induce Jerusalem by threats and promises to surrender. Spokesman for Tartan and Rabsaris. Possibly a Jewish deserter and apostate. This is favored by his familiarity with the Hebrew language, in which he addresses fluently (to the annoyance of Hezekiah's officers sent to meet him) the Jews on the wall, and with Isaiah's prophecy ( Isaiah 8:7-8;  Isaiah 10:5-6): "am I now come up without the Lord to destroy it? The Lord said, Go up against this land" ( 2 Kings 18:25). Isaiah ( Isaiah 33:14) alludes to traitors, "sinners in Zion," "hypocrites."

Rabshakeh was a zealous pleader for his master, reckless of truth, glossing over the real miseries of deportation by Assyria ( Isaiah 36:16-17), pretending to have Jehovah on his side, yet classing Jehovah with the idols of other lands overthrown by Assyria (  Isaiah 36:18-20 , Liars Need To Have Good Memories) , trying to rob the godly of their one only but sure trust in trouble, misrepresenting Hezekiah's faithful act in removing forbidden high places to Jehovah, as though he thereby had dishonored and so forfeited the favor of Jehovah ( Isaiah 36:7), boasting of Assyria's might, as if, because Judah could not supply 2,000 riders if even Assyria supplied the horses, it were impossible the Jews could repel one of the least of Assyria's captains ( Isaiah 36:8-9); in filthy and blasphemous language he threatens to reduce them to eat their own excrement in the extremity of famine ( Isaiah 36:12;  2 Chronicles 32:11): a sample of the true nature of the pagan attack on Jerusalem, at once arrogant, blasphemous, and reckless of all decency.

Morrish Bible Dictionary [2]

This is a title, signifying 'chief cup-bearer,' borne by an officer who was sent by Sennacherib with the Tartan (general) and a Rab-saris to Jerusalem. He was the chief spokesman; and from the fact of his being able to speak in the Jews' language, he is supposed to have been either a proselyte or an apostate Jew. If so he may possibly have been acquainted with  Isaiah 10:5,6 , for he says, "Am I now come up without the Lord against this place to destroy it? The Lord said to me, Go up against this land and destroy it."  2 Kings 18:17-37 . On the other hand, he profanely classes the God of Israel with all the gods that could not protect their worshippers from his master.  2 Kings 19:4,8;  Isaiah 36:2-22;  Isaiah 37:4-8 .

Smith's Bible Dictionary [3]

Rab'shakeh. (Chief Cupbearer).  2 Kings 19:1;  Isaiah 36:1;  Isaiah 37:1. One of the officers of the king of Assyria, sent against Jerusalem, in the reign of Hezekiah. See Hezekiah . (B.C. 713). The English version takes Rabshakeh as the name of a person; but it is, more probably, the name of the office which he held at the court, that of chief cupbearer.

People's Dictionary of the Bible [4]

Rabshakeh ( Răb'Ska-K Çh or Rab-Shâ'Keh ). An officer—the chief butler or cupbearer—who was sent with Rab-saris, the chief of the eunuchs, and Tartan, messengers of the king of Assyria, to Hezekiah, summoning him, in the most indecent and blasphemous manner, to surrender his capital.  2 Kings 18:17-37.

Watson's Biblical & Theological Dictionary [5]

a chief butler, or cupbearer. This is a term of dignity, and not a proper name. Rabshakeh was sent by Sennacherib, king of Assyria, to summon Hezekiah to surrender Jerusalem,  2 Kings 18:17-18;  2 Kings 19:4; Isaiah 36.

Hawker's Poor Man's Concordance And Dictionary [6]

RABSHAKEH, or Rab-Saces

A compound of words—Rab, master—Shakeh, cupbearer ( 2 Kings 18:17)

Holman Bible Dictionary [7]

 2 Kings 18:17-35

Mike Mitchell

Easton's Bible Dictionary [8]

 2 Kings 18:17-37

International Standard Bible Encyclopedia [9]

rab´sha - ke , rab - shā´ke ( רבשׁקה , rabhshāḳēh ): A compound word, the first part, rabh , indicating "head" or "chief" (see Rab-Mag; Rab-Saris ). The second part, which in the Aramaic, probably meant "cupbearer," had in this connection and elsewhere, according to later discoveries, an extended significance, and meant chief officer, i.e. chief of the heads or captains.

Rabshakeh was one of the officers sent by Sennacherib, the king of Assyria, with the Tartan and the Rabsaris to demand the surrender of Jerusalem, which was under siege by the Assyrian army ( 2 Kings 18:17 ,  2 Kings 18:19 ,  2 Kings 18:26 ,  2 Kings 18:27 ,  2 Kings 18:28 ,  2 Kings 18:37;  2 Kings 19:4 ,  2 Kings 19:8;  Isaiah 36:2 ,  Isaiah 36:4 ,  Isaiah 36:11 ,  Isaiah 36:12 ,  Isaiah 36:13 ,  Isaiah 36:22;  Isaiah 37:4 ,  Isaiah 37:8 ). The three officers named went from Lachish to Jerusalem and appeared by the conduit of the upper pool. Having called upon King Hezekiah, his representatives Eliakim, the son of Hilkiah, Shebnah, the scribe, and Joah, the recorder, appeared. Rabshakeh sent through them a message to the king in which he represented himself as the spokesman for the king of Assyria. He derided King Hezekiah in an insolent fashion in representing his trust in Egypt as a bruised reed which would pierce the hand. Likewise his confidence in Yahweh was vain, for He also would be unable to deliver them. Then the officers of the king replied, requesting him to speak in the Syrian language-which they understood, and not in the Jews' language which the people on the wall understood. This he refused to do, speaking still more loudly in order that they might hear and be persuaded. By bribery and appeal, by promise and by deception he exhorted them to turn traitor to Hezekiah and surrender to him. The people, however, true to the command of Hezekiah ( 2 Kings 18:36 ), "held their peace, and answered him not a word." Afterward Rabshakeh returned and "found the king of Assyria warring against Libnah". ( 2 Kings 19:8 ). From this description it is inferred that Rabshakeh was a man of considerable literary attainment, being able, in all probability, to speak in three languages. He had, in addition to his official power, dauntless courage, an insolent spirit and a characteristic oriental disregard for veracity.