Green’s Dictionary of Slang

(US black) boasting, insinuating; esp. in the form of a ritual game of testing a rival’s emotional strength by insulting their relatives; thus also as adj.

1926		Van Vechten Nigger Heaven 192: He caught phrases: posin’ an’ signifyin’, high-toned mustard-seed.
1944		D. Burley Orig. Hbk of Harlem Jive 146: Signifying—To say in a double-meaning manner.
1946		C. Himes ‘The Something in a Colored Man’ in Coll. Stories (1990) 406: ‘Lonesome?’ he cracked in a signifying voice.
1951		L. Brown Iron City 112: How come you don’t cut out all that signifying?
1964		R. Abrahams Deep Down In The Jungle 54: [footnote] The term ‘signifying’ seems to be characteristically Negro in use if not in origin. It can mean a number of things; in the case of the toast, it certainly refers to the monkey’s ability to talk with great innuendo, to carp, cajole, needle and lie. It can mean in other instances the propensity to talk around a subject, never quite coming to the point. It can mean ‘making fun’ of a person or situation. Also it can denote speaking with the hands and eyes [...] Thus it is ‘signifying’ to stir up a fight between neighbors, by telling stories; it is signifying to make fun of the police by parodying his motions behind his back; it is signifying to ask for a piece of cake by saying, ‘My brother needs a piece of that cake.’ It is, in other words, many facets of the smart-alecky attitude. [Ibid.] 71: Lion came back through the jungle more dead than alive, / When the monkey started more of the signifying jive.
1974		Peter Farb Word Play 122: The game begins when one youth ‘sounds’ another to see if he will play. That is done either by ‘signifying’ [...] or ‘the dozens’.
1980		J.L. Gwaltney Drylongso 91: I seen some crackuhs do the same numbah behin’ some stone signifyin’ and damn lyin’ of one of them jackleg peck preachuhs.
1988		R.R. Moore ‘Signifyin’ Monkey’ 🎵 But he drug his ass back to the jungle more dead than alive. / Just to run into that little monkey and some more of his signifying jive!
1990		S. Morgan Homeboy 132: ‘Nigger!’ hissed Champagne. ‘Shet up yer signifyin!’.
2012		D. Spivey ‘If You Were Only White’ 5: He excelled at the art of ‘playing the dozens,’ the old West African game of signifying [...] the art of the game being to top your opponent’s verbal jousts with superior comebacks.