bite of the cherry

A cherry is a small fruit usually eaten in a single bite, and the phrase was originally intended to refer specifically to an attempt to repeat an already-completed action. By the 1940s, cherry had taken on the additional meaning of "virginity" and the phrase was considered embarrassing, and so "apple" replaced "cherry" chiefly in American usage. As an apple is usually eaten in many bites, the additional nuance of "an unrepeatable action" was increasingly lost. The first recorded use of the "apple" form is in 1922.

来源：wiktionary