Definition & Meaning of "headcase"in English

What is the origin of the term "headcase" and how is it used?

Headcase shows up in mid-20th-century English as a casual insult for someone seen as mentally unstable, irrational, or hard to deal with. It's built from "head" meaning the mind, and "case" meaning a particular example or problem. Calling someone a "case" already suggested they were a problem; adding "head" narrowed it to their thinking or behavior. The word stuck because it sounds blunt and clinical without being technical, making it useful for everyday ridicule. It's still informal and dismissive, used to label someone as difficult or unbalanced rather than to describe an actual mental condition.