Green’s Dictionary of Slang

1. gonorrhoea.

1750

in Breslaw Tues. Club 233: Why is a man with a moderate pox like a finished church bell?...Because it is not without a Clapper [HDAS].

2. (US) a venereally diseased person.

1956

H. Gold Man Who Was Not With It (1965) 17: I used to join out the odds, had myself a stable of clappers for the gash-hounds too dumb to do their own howling.