lunatic
n. countablen. a person who acts in a very wild, foolish, or dangerous way. It is an old-fashioned word that can be quite rude if used to describe someone with a mental illness.
n. a person who is mentally ill, or one who behaves in an extremely reckless or foolish manner. Historically used in a medical or legal context, though now primarily used as a pejorative or informal descriptor for irrationality.
He drives like a lunatic on the highway.
The neighbors thought he was a lunatic for trying to build a giant boat in his tiny backyard.
From Middle English lunatik, from Old French lunatique, from Late Latin lunaticus (“moonstruck”), derived from Latin luna (“moon”), the connection stemming from the belief that changes of the moon caused intermittent insanity.
Often used metaphorically in modern English to describe reckless drivers or people with wild ideas.