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Phrase(s): So help me(, God)!
I so pledge and vow in God’s name. • I will do it. I really will, so help me, God!
McGrawhill's American Idioms And Phrasal Verbs
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Also, so help me God. I swear that what I am saying is true, as in So help me, I haven't enough cash to pay for the tickets, or I wasn't there, so help me God. This idiom became a formula for swearing a formal oath and is still so used in courts of law for swearing in a witness (I swear to tell the truth, the whole truth, and nothing but the truth, so help me God). It was first recorded in 1508 as "So help me, our Lord."
American Heritage Idioms