swear blind(redirected from swear up and black and blue)
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.