The Collaborative International Dictionary of English v.0.48:

Beau \Beau\ (b[=o]), n.; pl. F. Beaux (E. pron. b[=o]z), E.
   Beaus (b[=o]z). [F., a fop, fr. beau fine, beautiful, fr.
   L. bellus pretty, fine, for bonulus, dim. of bonus good. See
   Bounty, and cf. Belle, Beauty.]
   1. A man who takes great care to dress in the latest fashion;
      a dandy.
      [1913 Webster]

   2. A man who escorts, or pays attentions to, a lady; an
      escort; a suitor or lover.
      [1913 Webster]

WordNet (r) 3.0 (2006):

beau
    n 1: a man who is the lover of a girl or young woman; "if I'd
         known he was her boyfriend I wouldn't have asked" [syn:
         boyfriend, fellow, beau, swain, young man]
    2: a man who is much concerned with his dress and appearance
       [syn: dandy, dude, fop, gallant, sheik, beau,
       swell, fashion plate, clotheshorse]