The Collaborative International Dictionary of English v.0.48:

Nubile \Nu"bile\, a. [L. nubilis, fr. nubere to marry: cf. F.
   nubile. See Nuptial.]
   Of an age suitable for marriage; marriageable. --Prior.
   [1913 Webster]

   2. Sexually attractive, sometimes used as a genteel euphemism
      for having well-developed breasts; -- of a young woman.
      [PJC]

WordNet (r) 3.0 (2006):

nubile
    adj 1: of girls or women who are eligible to marry [syn:
           marriageable, nubile]