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swear blind

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swear blind

  (british & Australian) also swear up and down/black and blue (American & Australian/Australian)

to say that something is completely true, especially when someone does not believe you He swore up and down that he'd never seen the letter. If I ask her, I know she'll swear blind she locked the door. He swore black and blue he had nothing to do with the missing money.

See also: blind, swear