Think of a page pulled out of a magazine and pinned to a wall, so that anyone walking past can stop and look at it. Everything this word does starts from that wall.
可以想成一張從雜誌中間撕下來的紙，被釘在牆上，經過的人都會停下來看一眼。這個字的每個意思，都可以從那面牆開始想。
From the word’s history
Wiktionary records pin-up as a deverbal from the phrasal verb pin up, because the photograph was often cut out of a magazine and pinned up on a wall.
來源資料說 pin-up 是從動詞片語 pin up 直接變成的名詞，因為這種照片常常被從雜誌裡剪下來、釘到牆上；做這個動作的名字，後來就變成了那張照片的名字。
Wiktionary, "pin-up"
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