Down to fuck is a slang phrase that simply means that an individual is willing to engage in sexual intercourse. In conversation, the term is used as the acronym DTF as often as the full phrase is used.
Down to Fuck (DTF)
More About Down to Fuck (DTF)
The phrase is used by individuals to let others know that they are interested, but can also be a judgment-based opinion. Although an individual might announce that he or she is open to sex, as with all sexual encounters, consent should be verified and appropriate contraception used.