Green’s Dictionary of Slang

1. to steal, to pick pockets ‘after the approved method’ (Hotten 1859); thus knuckle a wipe, to steal a handkerchief; knuckling, picking pockets [SE knuckle; ‘the approved method,’ according to Vaux, implies the robbery of notes and cash rather than less valuable items].

1788		Grose Classical Dict. of the Vulgar Tongue (2nd edn) n.p.: Knuckle. [...] to steal pocket books, watches, &c.
1791		‘Flash Lang.’ in Confessions of Thomas Mount 19: Knuckling, picking of pockets.
1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 249: knuckle to pick pockets, but chiefly applied to the more refined branch of that art, namely, extracting notes, loose cash, &c., from the waistcoat or breeches pocket, whereas buzzing is used in a more general sense.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1788].
1834		Sun (N.Y.) 20 June 2/2: Off. — Well, what’s going on? Con. — Oh, nothing of consequence since I came out—only a little ‘knuckling’ just to keep my hand in. I can’t find a ‘Pal,’ and you know nobody can do a ‘Crack’ without one.
1848		‘Ned Buntline’ Mysteries and Miseries of N.Y. I 13: If you’d a been there you might have larnt ’em some lessons in the knuckling line?
1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1881		Newcastle Courant 2 Sept. 6/5: With my high character for knuckling, it was enough to be seen in a crowd to ensure my being taken before the queer cuffen if gripped.

2. (also knuck, knuckle up) to hit, to fight.

1811		Annotations in J. Poole Hamlet Travestie 100: The Billingsgate edition of the Slang Dictionary [...] explains it, to knuckle, or, to lather.
1856		Abbeville Banner (SC) 19 June 1/5: Knuckle him [...] make mince meat of him!!
1873		Sl. Dict. 211: Knuckle to fight with the fists, to pommel.
1946		C.S. Montanye ‘Don’t Meddle with Murder’ in Thrilling Detective May 🌐 He [...] pressed a finger over the place on his face where I’d knuckled him.
1953		F. Paley Rumble on the Docks (1955) 78: If you see a trouble-maker you knuckle him up.
1953		T.A.G. Hungerford Riverslake 169: He’s askin’ for trouble – somebody’ll knuckle him.
1967		P. Thomas Down These Mean Streets (1970) 6: [I] knuckled a bold but respectful noise on the door that sounded like a pat instead of a knock.
1974		‘P.B. Yuill’ Hazell Plays Solomon (1976) 10: They stopped booting and knuckling each other when they saw Old Bill.
1989		(con. 1950s–60s) in G. Tremlett Little Legs 2: He wasn’t a big man but he was game and he could knuckle.
1997		L. Pettiway Workin’ It 200: She want to knuck. She wanted me to fight her back.
1999		G. Seal Lingo 119: More terms concerned with the pugilistic include to knuckle, as in to knuckle someone.
2001		N. Barlay Hooky Gear 179: Then someone knuckle the door.
2013		A$AP Rocky ‘Suddenly’ 🎵 I’m so thuggish, ruggish, bringin’ ruckus, knuckin’ if you buckin’.