A "Clown Mask" is created when two men have a threesome with a woman and finish by ejaculating onto each others hands, high-fiving and then slapping the woman in the face simultaneously, after which one of them bops her on the nose. Giving the white-faced red-nosed appearance of a clown.
"Me and Pat gave Honey a clown mask last night."
Having unprotected sex with a girl when shes on her period than removing your penis and rubbing it on her nose, creating a red nose like a clown would have.
"Dude I totally clown masked a girl at that party last night!"