How is “so help me” used?

In How the West Was Won (1962), one line is: “...and so help me, the bucket was full of rocks.”

Which movie is “so help me” from?

The clip at the top of this page is from How the West Was Won (1962).

used to swear that what you are saying is true, especially in a court of law

Heard in 20 movies

Oxford100% of films
used to swear that what you are saying is true, especially in a court of law
So help me God.
It sounds crazy, Keyes, but it's true, so help me.

Definitions from the Oxford Advanced Learner's Dictionary, 9th edition

...and so help me, the bucket was full of rocks.

Said in 8 films · #1