making sharp shrill barking noises
c.1600, probably of imitative origin. Originally in reference to dog sounds; meaning "to talk idle chatter" is first recorded 1886. Related: Yapped; yapping.
The mouth; bazoo: every time you open your yap to say something (1900+)verb
To talk, esp idly or naggingly; yammer: You've been yapping away (1886+)Related Terms
[probably echoic, and similar to the sense ''yelp,'' esp as a small dog does]