The Act of ejaculating onto a woman's mustache (or ''Caterpillar'') thus allowing her to wipe away the applied semen, similar to a metamorphosis of a caterpillar to a butterfly.
Dan: I blew it all all over Susan's hairy upper lip.
Tony: Can you not say that about my mom!?
Dan: Fine but theres nothing wrong with giving a girl a Dirty Butterfly.