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.