What is the origin of impromptu?

The word "impromptu" comes from the Italian phrase "imprompt[u]" meaning "not prepared" or "unpremeditated".

It was first used in English in the 17th century and is a combination of the Latin prefix "in-" meaning "not" and the Italian word "promptu" meaning "ready" or "prepared".

Therefore, "impromptu" literally translates to "not ready" or "not prepared".