knock into a cocked hat

knock into a cocked hat idiom
Debunk, render useless or unbelievable. For example, His findings knocked our theory into a cocked hat. This expression alludes to a style of hat with the brim turned up on three sides—the three-cornered (tricorne) hat worn by officers in the American Revolution—giving it a distorted look. [Early 1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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