Definition
Illicit pleasure.
Origins
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)
How People Actually Use It
1. sex 2. Hot chick who never seems to go out with anyone. 3. A female that you know you can't become involved with past the point of friendship. This may be due to having a boyfriend/not being interested in you/or too busy to be involved with anyone. She's like the real forbidden fruit. You can't have a taste of her.
"Jane is Tim's forbidden fruit"
Source: Urban Dictionary