Technically speaking, a pin-up girl is a girl that men would usually buy a poster of up and pin to their wall or bunk. Historically, they were of famous celebrities, and they came to fame during World War II when soldiers would have posters of the pin-up girls.
Nowadays, pin-up girls have evolved into a bit more of a vague style, but they still consist of sexy girls in poses for posters than most guys would pin up.
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