The Collaborative International Dictionary of English v.0.48:

Bleat \Bleat\, v. i. [imp. & p. p. Bleated; p. pr. & vb. n.
   Bleating.] [OE. bleten, AS. bl?tan; akin to D. blaten,
   bleeten, OHG. bl[=a]zan, pl[=a]zan; prob. of imitative
   origin.]
   To make the noise of, or one like that of, a sheep; to cry
   like a sheep or calf.
   [1913 Webster]

         Then suddenly was heard along the main,
         To low the ox, to bleat the woolly train. --Pope
   [1913 Webster]

         The ewe that will not hear her lamb when it baas, will
         never answer a calf when he bleats.      --Shak.
   [1913 Webster]