Definition & Meaning of "dementoid"in English

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

Dementoid is a slang term that appeared in mid–20th-century English to describe a person considered mentally unstable, irrational, or foolish. The word combines "demented," meaning insane or mentally disturbed, with the suffix "-oid," implying likeness or resemblance, so it literally labels someone as being like a demented person. Historically, it was derogatory and insulting, used to mock or belittle someone for perceived mental instability or irrational behavior. Today it remains informal and offensive, generally limited to mocking, critical, or aggressive speech rather than polite or neutral contexts.