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Related to swear blind: swear up and down
To make a very serious, solemn pledge, especially that one is telling the truth. Janet has sworn blind that she wasn't the one to betray me, and I believe her.
swear blindBRITISH or
swear up and downAMERICAN
If someone swears blind that something is true, they insist that they are telling you the truth. He swore blind that he hadn't taken the money. He swears blind that he bears no grudges against Manchester United for sacking him, but I don't know if I believe him.
swear blindaffirm something in an emphatic manner. British informal
A North American variant of this expression is swear up and down .
swear ˈblind (that)...(informal) say that something is definitely true: She swore blind that she had not taken the money, and I believe her.
See also: swear