Green’s Dictionary of Slang

1. an incompetent, unskilled or unprincipled worker or professional person, esp. a quack doctor, a corrupt lawyer, a hypocritical preacher.

1853		‘Philip Paxton’ A Stray Yankee in Texas 284: In the Texan vocabulary, all men who have a mere inkling of any trade or profession are called ‘jack-legs.’ You will hear of ‘jack-leg’ lawyers, ‘jack-leg’ preachers, and ‘jack-leg’ doctors.
1908		L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:iv 323: jack-leg, n. A bungling workman, a cheat. [...] ‘They swallow every dost that’s give ’em by any jack-leg preacher that comes along.’.
1980		J.L. Gwaltney Drylongso xxiv: Just another jackleg educated fool. [Ibid.] 158: Except for some educated fools and jacklegs out here, when we talk to each other we talk so that we can be understood.
1988		(con. 1930s) C.E. Lincoln The Avenue, Clayton City (1996) 173: A sho’ nuff doctor is what you need. You don’t need no jackleg.

2. an itinerant preacher.

1934		Z.N. Hurston Jonah’s Gourd Vine (1995) 70: Here de pastor is. You ain’t nothin’ but uh jack-leg.
1980		J.L. Gwaltney Drylongso 221: One time the bukra preachuhs and they little black jacklegs said the moon was goin’ to trickle away into blood.
1988		(con. 1930s) E.C. Lincoln The Avenue, Clayton City (1996) 3: He yearned for a call to preach [...] [but] he would not let himself be lured into a self-called jackleg.