The Collaborative International Dictionary of English v.0.48:

Lassie \Las"sie\ (l[a^]s"s[i^]), n.
   A young girl; a lass. [Scot.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

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