Green’s Dictionary of Slang

1. the mouth.

1880		Leeds Times 14 Feb. 4/5: A fellow of the rough spcies, whom much loafing had made impudent [...] lifted uyp his yawp. ‘Say...’ [ etc.].
1927		(con. 1914–18) L. Nason Three Lights from a Match 236: If you open your yawp again, just once, I’ll wind this gun around your nose like a handkerchief.

2. loud or foolish chatter.

1889		Nat. Police Gaz. (NY) 7 Dec. 3/3: Chicago society circles have struck an impediment in their yawp, and all because of the fall of a pretty aud accomplished girl.
1913		Madison Jrnl (LA) 30 Aug. 3/3: Before I could finish my yawp, off he pelts.
1971		L. Bangs in Psychotic Reactions (1988) 85: They do sometimes approximate that nth devolution of the Fugs’ yawp.