Green’s Dictionary of Slang

(Aus./US) an ox-driver.

1859		Alta Calif. 17 Aug. n.p.: Gentile and Mormon, bull whacker and Pike’s Peaker, all seemed to mingle freely.
1861		Knickerbocker (N.Y.) lviii (Aug.) 117: Five yoke of oxen is the motive power for each wagon, and these are urged forward by a ‘bull-whacker’ armed with a whip carrying a lash from six to twelve feet in length.
1870		‘The Bull-Whacker’ in Lingenfelter et al. Songs of the Amer. West (1968) 59: We are bold bull-whackers on whom you can rely.
1878		J.H. Beadle Western Wilds 407: Numbers of ‘bull-whackers,’ sunburnt, healthy, and jolly, carrying with them their murderous whips.
1885		Arizona Champion (Mohave Co., AZ) 21 Feb. 1/5: The gentle-voiced teamster and the bull-whacker [...] congregated to listen.
1890		C.W. Haskins Argonauts of Calif. 52: We employed a Pike county bull-whacker, who agreed to deliver us and our effects in Hangtown.
1901		Bulletin (Sydney) 2 Nov. 36/2: ‘There’s only one thing,’ shouted Harry the Bull-Whacker (once in a cattle-stealing case) – ‘Harbitration – and I’ll be Judge!’.
1911		J. Lomax Cowboy Songs 69: I’m a lonely bull-whacker / On the Red Cloud line, / I can lick any son of a gun / That will yoke an ox of mine.
1922		M.E. Smith Adventures of a Boomer Op. 11: Johnson’s specialty was telling lies [...] he would have made a ‘Bullwhacker’ sound like a preacher when it came to that kind of talk.
1930		W. James Lone Cowboy 77: The bull-whacker had no use for horses and the skinner had no use for bulls.
1936		R.F. Adams Cowboy Lingo 199: An ox-team outfit making such of him a ‘bull-whacker’.
1954		Indep. Record (Helena, MT) 9 Oct. 3/7: A cowboy can be [...] a ‘bull whacker’ (if his chore consists of driving the camp’s supply wagons).