forbidden fruit
noun
: an immoral or illegal pleasure
Example Sentences
For dieters, dessert becomes a forbidden fruit.
Recent Examples on the Web Was an apple really the 'forbidden fruit' in the Garden of Eden? 
Paul Cappiello, The Courier-Journal, 16 Sep. 2022 Today both models cost about the same, averaging around $48,000, but prices continue to climb; a few years ago a great-condition R32 cost less than $30K. R34s, which will hit the magic 25-year mark in 2024, largely remain forbidden fruit. Scott Oldham, Car and Driver, 10 Nov. 2020
Paul Cappiello, The Courier-Journal, 16 Sep. 2022 Today both models cost about the same, averaging around $48,000, but prices continue to climb; a few years ago a great-condition R32 cost less than $30K. R34s, which will hit the magic 25-year mark in 2024, largely remain forbidden fruit. Scott Oldham, Car and Driver, 10 Nov. 2020 Word History
Etymology
from the forbidden fruit of the Garden of Eden in Genesis 3:2–19
First Known Use
1605, in the meaning defined above