The term 'dirty side burns' originated in the
porn industry, particularly films involving ass-to-mouth (
ATM) action. It describes the intentional
act of leaving a shit 'lamb chop side burns' on a partner's face by the dude taking his cock out of his partners ass and slapping it back and forth across her face without wiping it off
first. The result is that as the dick is slapped across her cheeks, shit it comes to rest on his partners face in a 'lamb chop side burns" configuration.
After he fucked her in the ass, he
smacked his dick back and forth across her face,
giving her dirty lamb chop side
burns.