A dirty richmond is an act performed during sexual intercourse. It begins by person A pouring cocaine into person B's butthole. Right before person A proceeds to snort the drug, person B lets out a fart, proceeding to blow a cloud of cocaine and the smell of ass into person A's face. Delighted by the act, person A the proceeds to huff in the mixture with a big deep breathe.
I walked in on Jack ripping a Dirty Richmond off Diane last night. The cloud was so big it fogged up the entire room!