Forbidden fruit
Definition 1 of 3
Pro Tip 1/3Cultural Reference
The term often alludes to themes of temptation and desire in literature and movies. 
In that film, the jewels were the forbidden fruit for the thieves.
Pro Tip 2/3Temptation Magnified
Desire for a 'forbidden fruit' is usually stronger because it's not allowed. 
Sneaking out at night was the forbidden fruit of his teenage years.
Pro Tip 3/3Not Literal Fruit
It's often not an actual fruit, but a metaphor for an enticing, banned thing. 
For her, the motorcycle was the forbidden fruit her parents warned her about.