mad as a hatter

idiomatic phrase

informal + old-fashioned

: severely mentally unsound

"Mad as a hatter," said the housekeeper. "Talks to himself, runs about waving his arms, won't come in for meals."

—Eva Ibbotson

"The whole thing is dashed fishy and mysterious and the sooner I can get Mildred safely out of the place, the better I shall be pleased. The fellow's as mad as a hatter!"

—P. G. Wodehouse

Word History

First Known Use

1827, in the meaning defined above