noun, plural fil·lies.
- a young female horse.
- Informal. a girl or young woman.
noun plural -lies
- a female horse or pony under the age of four
- informal, rare a spirited girl or young woman
c.1400, fyly, possibly from Old Norse fylja, fem. of foli “foal” (see foal (n.)). Slang sense of “young girl” is from 1610s.