The Collaborative International Dictionary of English v.0.48:

Bowwow \Bow"wow`\, n.
   An onomatopoetic name for a dog or its bark. -- a.
   Onomatopoetic; as, the bowwow theory of language; a bowwow
   word. [Jocose.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

bow-wow
    n 1: the bark of a dog
    2: informal terms for dogs [syn: pooch, doggie, doggy,
       barker, bow-wow]