Green’s Dictionary of Slang

capper n.1

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

1. [mid-18C+] (also cap) a confederate in a gambling game who poses as another gambler but actually works to swindle the genuine participants; similarly used in confidence tricks [such a confederate is always able to cap everyone else’s bet].

2. [late 19C+] (US) an employee of a casino, brothel, strip-club etc, who points a potential client towards the variety of self-indulgence they seek [ext. use of sense 1].

3. [late 19C+] anything seen as terminal, ‘the last straw’.

4. [1900s–20s] (US) a shop tout [ext. use of sense 1].

5. [1960s+] an anecdote that steals the limelight from a previous anecdote, a punchline.

6. (US black) one who triumphs in contests of verbal facility.