swear on

swear on (someone or something)

To make a very serious, solemn pledge or oath, especially that one is telling the truth. The person or thing following "on" is typically someone or something that one loves or respects. Janet has sworn on her mother's grave that she wasn't the one to betray me, and I believe her. I swear on my children that I will get revenge on the people that wronged you. You have to swear on the Bible when you testify in court.
See also: on, swear
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

swear (up)on someone or something

to take an oath on someone or something. (Upon is formal and less commonly used than on.) He swore upon the Bible to tell the truth. I swear on the memory of my sainted mother that I am telling the truth.
See also: on, swear
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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