Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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