Source : Webster's Revised Unabridged Dictionary (1913)

Yap \Yap\, v. i. [Icel. gj[=a]lpa; akin to yelp. Cf. {Yaup}.]
   To bark; to yelp. --L'Estrange.

Yap \Yap\, n.
   A bark; a yelp.

Source : WordNet®

yap
     n : informal terms for the mouth [syn: {trap}, {cakehole}, {hole},
          {maw}, {gob}]
     v : bark in a high-pitched tone; "the puppies yelped" [syn: {yelp},
          {yip}]
     [also: {yapping}, {yapped}]