(n)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)
The Collaborative International Dictionary of English (GCIDE) v.0.53Collaborative International Dictionary (GCIDE)
I know no spells, use no forbidden arts. Milton. [ 1913 Webster ]
Forbidden fruit. (a)Any coveted unlawful pleasure, -- so called with reference to the forbidden fruit of the Garden of Eden.(b) (Bot.) A small variety of shaddock (Citrus decumana). The name is given in different places to several varieties of Citrus fruits. [ 1913 Webster ]