The Collaborative International Dictionary of English v.0.48:

Baa \Baa\ (b[aum]), v. i. [Cf. G. b[aum]en; an imitative word.]
   To cry baa, or bleat as a sheep.
   [1913 Webster]

         He treble baas for help, but none can get. --Sir P.
                                                  Sidney.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Baa \Baa\ (b[aum]), n.; pl. Baas (b[aum]z). [Cf. G. b[aum].]
   The cry or bleating of a sheep; a bleat.
   [1913 Webster]

WordNet (r) 3.0 (2006):

baa
    n 1: the cry made by sheep
    v 1: cry plaintively; "The lambs were bleating" [syn: bleat,
         blate, blat, baa]