When a chick is mean muggin, she may be down to fuck, but she is definitely down to frown.
Frat boy #1: Dude that girl is totally DTF!
Frat boy #2: Nah man, I think she just broke up with her boo... she doesn't want your curved penis.
Frat boy #1: Down to frown man, down to frown.
Example:
When someone is annoying a group of friends, and they want to get rid of him, they can use it in "code" to tell each other in quick they don't like that extra person.
Annoying person: And this one time, at band camp...
Friend 1 to Friend 2: Dude DTF
Friend 2 to Friend 1: Done ;)