Where did the phrase "O Brother, Where Art Thou" come from?

I just started watching the 1941 comedy film Sullivan's Travels. About 5 minutes in, the lead (played by actor Joel McCrea) asks one of the producers "What do you want me to do, make O Brother, Where Art Thou?

The producer answers "Yes...."

Anyone have a handle on where this reference comes from? Obviously he's not referring to the George Clooney flick. 😆 and yes, I did a few Google searches. Also wondering if this is where the Coen brothers got the idea for the title?

I really like the movie, btw

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