The Collaborative International Dictionary of English v.0.48:

Tomboy \Tom"boy`\, n. [Tom (for Thomas, L. Thomas, fr. Gr. ? )+
   boy.]
   A romping girl; a hoiden. [Colloq.] --J. Fletcher.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tomboy
    n 1: a girl who behaves in a boyish manner [syn: tomboy,
         romp, hoyden]