A prohibited article.
Forbidden fruit originates from the Garden of Eden bible story. The biblical 'forbidden fruit' was of course the apple. In the story the type of fruit isn't actually mentioned - God forbade Adam and Eve to touch the fruit of the tree of knowledge. It is widely interpreted as being an apple though and the 'Adam's apple' is named after the fruit which is supposed to have stuck in Adam's throat.
Genesis 2:9 (King James Version) And out of the ground made the LORD God to grow every tree that is pleasant to the sight, and good for food; the tree of life also in the midst of the garden, and the tree of knowledge of good and evil.
2:16 And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat:
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.
3:3 But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die.
These days apples are hardly forbidden - we are positively encouraged to eat them.
The term began to be used figuratively in the 17th century. In 1663, Heath used it in his Flagellum; or, the Life and Death of Oliver Cromwell:
"The stealing and tasting of the forbidden fruit of Soveraignty."
These days fruit is, far from being forbidden, pressed on us by every nutritionist since 'an apple a day keeps the doctor away' was coined.