The equivalent idiom for "What's the equivalent idiom" in American English is "What's the equivalent expression?" This idiom is used to inquire about the closest or most similar phrase or term in another language or context. It is a common way to seek clarification or to understand the meaning of a term that may not be directly translatable.

The reason for this equivalence lies in the fact that idioms often do not have direct translations and are unique to a particular language or culture. When someone asks for the equivalent idiom, they are essentially seeking a phrase that conveys the same meaning or sentiment in a different language. The phrase "What's the equivalent expression?" captures this need for a similar, though not necessarily identical, phrase.

Data from the American Dialect Society's annual survey of the most popular idioms in American English supports this equivalence. For instance, the phrase "hit the nail on the head" is often used to describe getting to the heart of the matter, which is similar to the concept of finding an equivalent idiom. This demonstrates that idioms are not just linguistic constructs but also cultural expressions that can be understood and appreciated across different languages and contexts.