A mustache on the genitals. With the penis acting as a nose, the nut stache is the pubic hair under the shaft. It also may be part of a full nut beard.
You have such a thick nut stache!
When you finish a blow job by cumming on her upper lip, like a milk mustache.
... turns out she doesn't swallow so she ended up with a Nut Stache... looked like President Taft.