D double T aka DTT stands for Down to Train. Which a male could say to a person of the same or opposite sex. It would be used to tell how a guy or girl is ok with having multiple sex partners in one night, not at the same time but one after another. Therefor he or she is down to train, as to run a train through her or him.
Guy 1:''damn dude, check that chick out she's so hot!''
Guy 2:''and she is so all over every guy at this party''
Guy 1:''she is totally D double T man! Lets get a line going!!''