The Collaborative International Dictionary of English v.0.48:

Yak \Yak\ (y[a^]k), n. [Tibetan gyag.] (Zool.)
   A bovine mammal (Poephagus grunnies) native of the high
   plains of Central Asia. Its neck, the outer side of its legs,
   and its flanks, are covered with long, flowing, fine hair.
   Its tail is long and bushy, often white, and is valued as an
   ornament and for other purposes in India and China. There are
   several domesticated varieties, some of which lack the mane
   and the long hair on the flanks. Called also chauri gua,
   grunting cow, grunting ox, sarlac, sarlik, and
   sarluc.
   [1913 Webster]

   Yak lace, a coarse pillow lace made from the silky hair of
      the yak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

yak
    n 1: noisy talk [syn: yak, yack, yakety-yak, chatter,
         cackle]
    2: large long-haired wild ox of Tibet often domesticated [syn:
       yak, Bos grunniens]
    v 1: talk profusely; "she was yakking away about her
         grandchildren" [syn: yak, gab]

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "yak":
   Brahman, Indian buffalo, aurochs, babble, babblement,
   bat the breeze, bavardage, beat the gums, beef, beef cattle,
   beeves, bibble-babble, bison, blab, blabber, blah-blah, blather,
   blether, blethers, bossy, bovine, bovine animal, buffalo, bull,
   bullock, cackle, calf, caquet, caqueterie, carabao, cattle, chat,
   chatter, chin, chitter-chatter, clack, clatter, cow, crack,
   critter, dairy cattle, dairy cow, dither, dogie, gab, gabble, gag,
   gas, gibber, gibble-gabble, go on, gossip, guff, gush, haver,
   heifer, hornless cow, hot air, idle talk, jabber, jape, jaw, jest,
   kine, leppy, maverick, mere talk, milch cow, milcher, milk cow,
   milker, muley cow, muley head, musk-ox, natter, neat,
   nonsense talk, ox, oxen, palaver, patter, pour forth, prate,
   prating, prattle, prittle-prattle, quip, ramble on, rattle,
   rattle on, reel off, run on, shoot the breeze, spiel, spout,
   spout off, steer, stirk, stot, talk away, talk nonsense, talk on,
   talkee-talkee, tittle-tattle, twaddle, twattle, waffle, waggery,
   wisecrack, wisent, witticism, yakkety-yak, yap, yearling, zebu