Green’s Dictionary of Slang

cap v.5

[early 19C dial. (later SE) cap, to surpass, to outdo]

1. to make a smart rejoinder.

[Aus]Bulletin (Sydney) 2 Aug. 36/1: Big John O’Shanassay (Vic.) minted a long-lived lie with his phrase about them ‘walking about looking for work and praying to God they may never find it.’ Governor Du Cane, of Tasmanaia, capped that when he told a London audience that ‘on many big Australian stations the cost of feeding travellers was more than all the other expenses put together.’.
[Ire]Joyce Ulysses 196: He died dead drunk, Buck Mulligan capped.
[US]Mezzrow & Wolfe Really the Blues 219: Listen at ole Mezz, cappin’ and on time.
[US]L. Bruce Essential Lenny Bruce 235: Guys would cap each other.
[US](con. 1940s–60s) H. Huncke ‘Sea Voyage’ in Eve. Sun Turned Crimson (1998) 142: Finally capping himself when he suddenly exclaimed, while observing a large, almost perfectly round puff of black smoke [...] ‘Oh look — it’s just like a big dinge nut.’.

2. to lie.

[US]A.H. Lewis Confessions of a Detective 36: Didn’t I cap for you, an’ square you with the examinin’ board?
[US]Ebonics Primer at [Internet] cappin’ Definition: to lie to a great extent Example: I can’t believe dat nigga said he got the 2010 BMW! He’s straight cappin’!

3. to brag about, to aggrandize.

[US]H. Green Actors’ Boarding House (1906) 109: I ain’t a gell that’s always cappin’ her own game.

4. (US black) to surpass, to outdo [note 19C+ SE use with same meaning; however, this sl. US black use is interlinked with sense 7].

[US]Cab Calloway New Hepsters Dict. in Calloway (1976) 253: capped (v.): outdone, surpassed.
[US] ‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.
[US]R.S. Gold ‘Vernacular of the Jazz World’ in AS XXXII:4 277: capped. Excelled.
[US]F. Salas Tattoo the Wicked Cross (1981) 129: You can’t cap me [...] ’cause I come from way down West.
[US]D. Claerbaut Black Jargon in White America 60: capped v. outdid; did better, often referring to a verbal exchange: He capped you.

5. to dumbfound, to render silent.

[UK]K. Waterhouse Billy Liar (1962) 91: ‘By, that’s capped me theer, Councillor!’ ‘Aye, and tha’s capped me an all.’.

6. (US, also cap on) to entice a victim into a swindle.

[US]‘Iceberg Slim’ Trick Baby (1996) 120: Both of us could cap on or build up a sucker.

7. (US black, also cap on) to insult someone, esp. by disparaging their family.

L. Bruce Lenny Bruce: I Am Not a Nut [album] It would be very different if the sharks were flagrant offenders, but, I mean, they made one mistake and everybody capped on them immediately.
[US](con. 1950s) D. Goines Whoreson 39: She would only laugh and cap.
[US]E. Folb Runnin’ Down Some Lines 232: cap (on one) 1. See base. 2. Discredit another (often done in a joking way). 3. Make fun of, embarrass.
[US]Too $hort ‘Cusswords’ [lyrics] If I ever said a rap, tryin to cap on you / I wouldn’t even sweat it ’cause you’ll be through.
[US]P. Munro Sl. U. 54: Joe’s friends are going to totally cap on him when they find out he is going out with a thirteen-year-old girl.
[US]Da Bomb [Internet] 6: Cap (cap on): To insult, usually for fun; making fun of.
[US]Teen Lingo: The Source for Youth Ministry [Internet] cap v. To insult or make fun of. ‘Why you always cappin’ on me? Don’t make me open up a can on you!’.
[US]Prison Slang Mommyblogger 2 Mar. [Internet] That knick-knack needs to get mud checked for getting on your leg and then cappin’ it with no explanation.