yap v.

1660s, "bark as a (small) dog," earlier as a noun, "yapping dog" (c. 1600), probably of imitative origin. Compare verb yamph in same sense (1718). Originally in reference to dog sounds; meaning "to talk idle chatter" is first recorded 1886. Related: Yapped; yapping. As a noun, 1826 in reference to the sound; 1900, American English slang as "mouth."