This raises the question of why we call girls "chicks" at all. Etymonline gives
Quote:
c.1320, abbreviation of chicken (q.v.), extended to human offspring (often in alliterative pairing chick and child) and used as a term of endearment. As slang for "young woman" it is first recorded 1927 (in "Elmer Gantry"), supposedly from U.S. black slang, in British use by c.1940, popularized by Beatniks late 1950s. Chicken in this sense is from 1711."
Which really doesn't give too much elucidation, aside from maybe that it comes from the "term of endearment" usage.
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* here for the nachos.