When you finish giving someone a blow job, then when they cum, it lands on your face making the jizz look like a mustache.
Your mom after she gives your dad head, and she says she was drinking milk, or brushing her teeth. It really is a jizzstache.
When a guy ejaculates onto a women's face and it goes above her lip, taking the form of a white moustache. Clean Sanchez.
Guy: hey you got something on your face.
Girl: what, do I got milk?
Guy: you got jizz.
Girl: damnit, why'd you give me a jizzstache!