1877, "small, tiny, for children," a dialect word, possibly a varied reduplication of wee. Attested earlier (1848) as a noun meaning "a small marble." (Baseball Hall-of-Famer Harold "Peewee" Reese got his nickname because he was a marbles champion before he became a Dodgers shortstop.) As a type of bird (variously applied on different continents) it is attested from 1886, imitative of a bird cry.
A short or small person, animal, etc: That peewee doesn't scare me (1877+)