Green’s Dictionary of Slang

yap n.1

[SE yap, a yelping dog; or its bark]

1. pertaining to speech.

(a) [mid-19C+] (US, also yapper, yop) the mouth, usu. in derog. sense, e.g. shut your yap! etc.

(b) [mid-19C+] (orig. US, also yop) idle, trivial chatter; an act of speaking (see cit. 1940).

(c) [20C+] (Irish, also yerp) a whining, complaining person.

(d) [20C+] (US, also yapper, yap yap) a chatterer.

(e) [1910s–20s] (US, also yapper) a tell-tale, an informer.

(f) [1920s–50s] a sound.

(g) [1950s+] (orig. US) a chat, a conversation.

(h) [1980s] a petty swindler.

2. fig., derog. uses .

(a) [late 19C–1940s] (US, also yappo) a contemptible person, irrespective of class or background.

(b) [late 19C+] (US, also yawp) a derog. term for a peasant, a rustic simpleton.

(c) (also yap-yap) attrib. use of sense 2b.

(d) [20C+] (US Und.) a criminal’s victim.

(e) [1920s] (US tramp) a novice within the hobo community.

In compounds

yap asylum (n.)

a night club.

yap wagon (n.)

[1920s] (US) a vehicle taking tourists on sightseeing tours.

In phrases

clap one’s yap (v.)

[1950s] (US) to be quiet.

sling the yap (v.)

to chatter.