The term "forbidden fruit" refers to any object of desire that's tempting precisely because we're not supposed to want it.

The expression comes from the Bible's Book of Genesis. You know, the story about how Adam and Eve were kicked out of the Garden of Even after defying God by eating fruit from the Tree of Knowledge of Good and Evil.

Even though the forbidden fruit is thought of as an apple, the Bible doesn't specify. But the apple is merely a metaphor for anything that is off-limits for moral, ethical, legal or any other reasons.