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]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "tomboy":
   amazon, androgyne, babe, baby, broad, butch, chick, colleen, cutie,
   dame, damoiselle, damsel, demoiselle, doll, dyke, filly, frail,
   gal, gamine, girl, girlie, heifer, hoyden, jeune fille, jill,
   junior miss, lass, lassie, lesbian, little missy, mademoiselle,
   maid, maiden, miss, missy, nymphet, piece, romp, schoolgirl,
   schoolmaid, schoolmiss, skirt, slip, subdeb, subdebutante, subteen,
   subteener, teenybopper, tomato, virago, virgin, wench,
   young creature, young thing