Dhorns ???

Saw Louie Anderson at Treasure Island Casino a couple weeks back. In his stand up he mentioned how his father used to call people “Dhorns” if he didn’t like them. An older women behind me remarked that her father did the same thing. The overwhelmingly older crowd roared when louie mentioned this. Anyone know the origination of the word? How common of a term was this? Or was it more specific to us Minnesotans/Midwesterns?

Dhorns = devil horns?

How do you know how the word was spelled if you just heard it said?