1
general::
Phrase(s): swear on a stack of Bibles [and] swear on one’s mother’s grave
to state something very earnestly, pledging to tell the truth. (~ a stack of Bibles refers in an exaggerated way to swearing to tell the truth in court by placing one’s hand on a Bible.) • I swear on a stack of Bibles that I am telling the truth. • Of course, I’m telling the truth. I swear on my mother’s grave!
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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]
American Heritage Idioms