Hah yeah it's a sleazy phrase skanky guys use when they're trying to be cool.
It basically means "Down to f**k?" and is like saying, "hey are you free to engage in casual intercourse even though I have no interest in showing you any respect and don't want to waste time getting to know you.
Basically, I don't respond well to dtf messages unless the guy is like really really hot, and it's more likely to end in a slap then sex most times.
I guess you can also use dtf as a phrase to let someone else know you're available. See example here: