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]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "beau":
   Beau Brummel, Casanova, Don Juan, Lothario, Romeo, amoroso,
   beloved, blade, blood, boulevardier, boyfriend, caballero,
   cavalier, cavaliere servente, clotheshorse, coxcomb, dandy, dude,
   esquire, exquisite, fashion plate, fellow, fine gentleman, flame,
   fop, fribble, gallant, gentleman friend, gigolo, inamorato,
   jack-a-dandy, jackanapes, lady-killer, love-maker, lover, macaroni,
   man, man-about-town, masher, necker, old man, petter, philanderer,
   puppy, seducer, sheik, spark, sport, squire, steady, sugar daddy,
   swain, sweetheart, swell, truelove, young man