nutty as a fruitcake
Crazy, idiotic, as in Mary's nutty as a fruitcake if she thinks she can get away with that. The adjective nutty meaning “insane” was first recorded in 1821; the similarity to fruitcake, which literally contains nuts as well as fruit, was first recorded in 1935.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.