The Collaborative International Dictionary of English v.0.48:

Lass \Lass\ (l[.a]s), n. [OE. lasse; prob. of Celtic origin; cf.
   W. llodes girl, fem. of llawd lad. [root]123. See Lad a
   youth.]
   A young woman; a girl; a sweetheart.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lass
    n 1: a girl or young woman who is unmarried [syn: lass,
         lassie, young girl, jeune fille]