Merriam-Webster Collegiate Dictionary - yak

 I. noun  (plural ~s; also ~)  Etymology: Tibetan (Lhasa dialect) ya? (spelled gyag  Date: 1795 a large long-haired wild or domesticated ox (Bos grunniens syn. B. mutus) of Tibet and adjacent elevated parts of central Asia  II. variant of yuk I  III. intransitive verb  also yack  (~ked; also yacked; ~king; also yacking)  Etymology: probably imitative  Date: 1949 to talk persistently ; chatter  IV. noun  also yack  Date: 1950 persistent or voluble talk