pin-up (adj.)

1670s, of clothing, "adapted for being pinned up," from the verbal phrase (attested from mid-15c. in the sense "affix in a prominent place"), from pin (v.) + up (adv.). From 1940, in reference to pictures of "winsome young ladies in daring undress" ("Life," May 6, 1940) such as soldiers pinned up on their dugout walls, etc. The thing itself is older than the name. The noun in this sense is recorded from 1943.

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Definitions of pin-up from WordNet

pin-up (n.)
a very pretty girl who works as a photographer's model;
Synonyms: cover girl / lovely