The Collaborative International Dictionary of English v.0.48:

Filly \Fil"ly\, n.; pl. Fillies. [Cf. Icel. fylia, fr. foli
   foal. See Foal.]
   1. (Zool.) A female foal or colt; a young mare. Cf. Colt,
      Foal.
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            Neighing in likeness of a filly foal. --Shak.
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   2. A lively, spirited young girl. [Colloq.] --Addison.
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