The Collaborative International Dictionary of English v.0.48:

Yap \Yap\, v. i. [Icel. gj[=a]lpa; akin to yelp. Cf. Yaup.]
   To bark; to yelp. --L'Estrange.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Yap \Yap\, n.
   A bark; a yelp.
   [1913 Webster]

WordNet (r) 3.0 (2006):

yap
    n 1: informal terms for the mouth [syn: trap, cakehole,
         hole, maw, yap, gob]
    v 1: bark in a high-pitched tone; "the puppies yelped" [syn:
         yelp, yip, yap]

Moby Thesaurus II by Grady Ward, 1.0:

168 Moby Thesaurus words for "yap":
   air a grievance, backwoodsman, bark, bat the breeze, battle cry,
   bawl, bay, bazoo, beat the gums, beef, bell, bellow, bellyache,
   bitch, blare, blat, blate, bleat, blubber, boom, bray, breathe,
   bug, bumpkin, buzz, cackle, call, carp at, caterwaul, chant, chaps,
   cheer, chin, chirp, chops, clamor, clodhopper, clown, complain,
   coo, crab, croak, crow, cry, drawl, embouchure, exclaim, flute,
   fret, fret and fume, fret at, fuss, fuss at, gab, gasp,
   give tongue, give voice, gob, gripe, groan, grouch, grouse, growl,
   grumble, grunt, hail, halloo, hassle, hayseed, henpeck, hick, hiss,
   holler, hollo, hoot, howl, hurrah, jaw, jaws, jowls, keen, kick,
   kisser, lilt, lips, lodge a complaint, low, mandibles, maw,
   maxilla, meow, mew, mewl, miaow, moo, mouth, mug, mumble, murmur,
   mush, mutter, muzzle, nag, neigh, nibble at, nicker, niggle,
   oral cavity, pant, peck at, pester, pick at, pick on, pipe,
   premaxilla, provincial, pule, raise a howl, rallying cry,
   register a complaint, roar, row, rumble, screak, scream, screech,
   shoot the breeze, shout, shriek, sibilate, sigh, sing, snap, snarl,
   snort, sob, spiel, squall, squawk, squeak, squeal, take on,
   thunder, trap, troat, trumpet, twang, ululate, wail, war cry,
   war whoop, warble, whicker, whine, whinny, whisper, whoop, yak,
   yakkety-yak, yammer, yap at, yawl, yawp, yell, yelp, yip, yo-ho,
   yowl