Forbidden fruit is a name given to the fruit growing in the Garden of Eden which God commands mankind not to eat. In the biblical story, Adam and Eve ate the fruit from the tree of the knowledge of good and evil and are exiled from Eden:
And the Lord God commanded the man, saying, Of every tree of the garden thou mayest freely eat:
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.
— Genesis 2:16–17, King James Version[1]
As a metaphor outside of the Abrahamic religions, the phrase typically refers to any indulgence or pleasure that is considered illegal or immoral.