swear on a stack of Bibles
Promise solemnly that what one is about to say is true, as in I swear on a stack of Bibles that I had nothing to do with his dropping out. This term alludes to the practice of placing one's hand on a sacred object while taking an oath, which dates from the mid-10th century. It is still followed in courts of law where a witness being sworn to tell the truth places a hand on the Bible. [Mid-1800s]
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.