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Phrase(s): forbidden fruit
Fig. someone or something that one finds attractive or desirable partly because having the person or thing is immoral or illegal. (Biblical; from the apple in the Garden of Eden that was forbidden to Adam by God.) • Jim flirts with his sister-in-law only because she’s forbidden fruit. • The boy watches that program only when his parents are out. It’s forbidden fruit.
McGrawhill's American Idioms And Phrasal Verbs