Slang mainly referring to simply hardcore dirty sex, where the amenities aren't followed.
Slang for where the sexual act or acts, are done merely for the physical pleasure of the parties. There is no foreplay, as a rule, or at least in the more accepted sense.
It is about rough sex, to a degree, where there may not be any preliminary leading up to anal sex, or such. Usually simply a reference to letting one's instincts and desires take charge.
Can also be in reference to whether safer sex practices are followed or not. Could be considered derogatory in that sense, associating bareback sex with being dirty, or obscene, or immoral.