Definition & Meaning of "flatline"in English

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

Flatline is a slang term that appeared in late 20th-century English to describe a person so stupid or incapable that they are effectively "brain-dead." The term comes from medical monitors showing a flat line when someone's heart stops or there is no brain activity, and it was applied metaphorically to people who seem completely unthinking or incompetent. Historically, it was derogatory, used to insult or ridicule someone's intelligence or ability. Today it remains informal and insulting, used in mocking or critical contexts rather than polite or neutral conversation.