Filly \Fil"ly\, n.; pl. Fillies. [Cf. Icel. fylia, fr. foli foal. See Foal.]

1. (Zo["o]l.) A female foal or colt; a young mare. Cf. Colt, Foal.

Neighing in likeness of a filly foal. --Shak.

2. A lively, spirited young girl. [Colloq.]

--Addison.

Source: Webster's Revised Unabridged Dictionary (1913)