A person with strange sexual urges, fantasies, and behavior involving objects, suffering or humiliation, or nonconsenting partners. Often has a scottish accent.
when a guy sticks his cock in boiling hot shit and then slaps people with it, hence leaving a "dirty mark". Typically done by the young gaycommunity in the UK.
John: "I had a lovely time getting bummed by you yesterday alex."
Alex:" haha thanks, if you look in the mirror youll see the results of a dirty mark"