A person you wouldn't typically have sex with but you do because she/he is there and available. (not hot but not ugly enough to pass up, better then masturbation)
Similar to how you hit an animal in the raod, not really because you wanted to but you didnt really not want to at the same time. why avoid it??
Friend: "hey check out that chick over there, she's looking at you"
You: "eh, banging body but she's got a big nose and kankles, she's Road Kill. We'll see how the rest of the night goes first"
Friend: "f*ck that dude, you know damn well if you dont hit that your going to go home alone and beating off to that free 30 second porn on the internet"