While at a social event, it is the guy/girl you know you can take home at the end of the night if you don't score with the girl/guy that you really want to hit. Usually of mediocre looks/intelligence/sex appeal.
The person with whom you will settle having sex with if your date decides 'I'm not that kind of girl/guy' and won't put out.
I was set to nail that MILF I met at the footbal game, but then her fuckin' kid sprained his ankle and couldn't spend the night at his friends house. I had to call that last resort slut on the skank block!