A butternut is when you use butter as a lubricant, then cum into a smallcrater in the butter and freeze it over night which then the next day you will eat it while it is still frozen
1. A completely unattractive woman that may have man-like features. Analagous to butterface, but seems like she might also possess testicles. Everything is attractive "but-her-nut."
Cain: She's really hot!
Abel: That chick's got a beard! She's a butternut.