When one masterbates with a dildo(originally with wooden dildo, but this is not necessary), up his/her (usually male and must have moustache)anus, and then pulls it out and whipes it on his moustache. This is known as a dirty moffatt.
When a man (not neccessarily but usually a tech teacher) takes a dildo, (usually made of wood) and sticks it up his ass while masterbating. After coming, the man pulls out the dildo, and then, with a left-to-right motion, whipes it on his moustache.
bradly gave himself a dirty moffatt on the snowday.