Is Blockhead an Idiom?

One who is stupid or foolish. Geez, he keeps pulling on a door that's clearly marked "push"—what a blockhead.

Is it an idiom or a idiom?

An idiom is a phrase or expression that typically presents a figurative, non-literal meaning attached to the phrase; but some phrases become figurative idioms while retaining the literal meaning of the phrase. Categorized as formulaic language, an idiom's figurative meaning is different from the literal meaning.

What is an idiom called?

An idiom (also called idiomatic expression) is an expression, word, or phrase that has a figurative meaning conventionally understood by native speakers. This meaning is different from the literal meaning of the idiom's individual elements. ... They have, however, a hidden meaning.