Green’s Dictionary of Slang

cap v.5

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

1. [20C+] to make a smart rejoinder.

2. [20C+] to lie.

3. [1900s] to brag about, to aggrandize.

4. [1930s+] (US black) to surpass, to outdo [note 19C+ SE use with same meaning; however, this sl. US black use is interlinked with sense 7].

5. [1950s] to dumbfound, to render silent.

6. [1960s] (US, also cap on) to entice a victim into a swindle.

7. [1960s+] (US black, also cap on) to insult someone, esp. by disparaging their family.