DMR or dmr is short for "Din mamma rövhål" or in english "Your mums asshole". It's often used between friends but like the saying Joe Mama it can also be used on people who don't know what it is.
Swedish:
Person 1 " Var ska vi äta?"
Person 2 "Dmr hade varit gott."
Person 1 "Vad är Dmr?"
Person 2 "Din mamma rövhål!!!"
English:
Person 1 " Where should we eat?"
Person 2 "I want really want dmr right now."
Person 1 "What's dmr?"
Person 2 "Your mums asshole!!!"