The Collaborative International Dictionary of English v.0.48:

Jehu \Je"hu\, n. [From Jehu, son of Nimshi. 2 Kings ix. 20.]
   A coachman; a driver; especially, one who drives furiously.
   [Colloq.]
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Jehu
   Jehovah is he. (1.) The son of Obed, and father of Azariah (1
   Chr. 2:38).

     (2.) One of the Benjamite slingers that joined David at Ziklag
   (1 Chr. 12:3).

     (3.) The son of Hanani, a prophet of Judah (1 Kings 16:1, 7; 2
   Chr. 19:2; 20:34), who pronounced the sentence of God against
   Baasha, the king of Israel.

     (4.) King of Israel, the son of Jehoshaphat (2 Kings 9:2), and
   grandson of Nimshi. The story of his exaltation to the throne is
   deeply interesting. During the progress of a war against the
   Syrians, who were becoming more and more troublesome to Israel,
   in a battle at Ramoth-gilead Jehoram, the king of Israel, had
   been wounded; and leaving his army there, had returned to
   Jezreel, whither his ally, Ahaziah, king of Judah, had also gone
   on a visit of sympathy with him (2 Kings 8:28, 29). The
   commanders, being left in charge of the conduct of the war, met
   in council; and while engaged in their deliberations, a
   messenger from Elisha appeared in the camp, and taking Jehu from
   the council, led him into a secret chamber, and there anointed
   him king over Israel, and immediately retired and disappeared (2
   Kings 9:5, 6). On being interrogated by his companions as to the
   object of this mysterious visitor, he informed them of what had
   been done, when immediately, with the utmost enthusiasm, they
   blew their trumpets and proclaimed him king (2 Kings 9:11-14).
   He then with a chosen band set forth with all speed to Jezreel,
   where, with his own hand, he slew Jehoram, shooting him through
   the heart with an arrow (9:24). The king of Judah, when trying
   to escape, was fatally wounded by one of Jehu's soldiers at
   Beth-gan. On entering the city, Jehu commanded the eunchs of the
   royal palace to cast down Jezebel into the street, where her
   mangled body was trodden under foot by the horses. Jehu was now
   master of Jezreel, whence he communicated with the persons in
   authority in Samaria the capital, commanding them to appear
   before him on the morrow with the heads of all the royal princes
   of Samaria. Accordingly on the morrow seventy heads were piled
   up in two heaps at his gate. At "the shearing-house" (2 Kings
   10:12-14) other forty-two connected with the house of Ahab were
   put to death (2 Kings 10:14). As Jehu rode on toward Samaria, he
   met Jehonadab (q.v.), whom he took into his chariot, and they
   entered the capital together. By a cunning stratagem he cut off
   all the worshippers of Baal found in Samaria (2 Kings 10:19-25),
   and destroyed the temple of the idol (2 Kings 10:27).

     Notwithstanding all this apparent zeal for the worship of
   Jehovah, Jehu yet tolerated the worship of the golden calves at
   Dan and Bethel. For this the divine displeasure rested upon him,
   and his kingdom suffered disaster in war with the Syrians (2
   Kings 10:29-33). He died after a reign of twenty-eight years
   (B.C. 884-856), and was buried in Samaria (10:34-36). "He was
   one of those decisive, terrible, and ambitious, yet prudent,
   calculating, and passionless men whom God from time to time
   raises up to change the fate of empires and execute his
   judgments on the earth." He was the first Jewish king who came
   in contact with the Assyrian power in the time of Shalmaneser
   II.


Hitchcock's Bible Names Dictionary (late 1800's):

Jehu, himself who exists