Forbidden fruit
WordNet
noun
(1) Originally an apple from the tree of knowledge of good and evil in the Garden of Eden; it is now used to refer to anything that is tempting but dangerous (as sexuality)
WiktionaryText
Etymology
With reference to Genesis 2:17: ‘But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.’ (KJV)