The girl that is the easy, last resort fuck at the bar or party. You go around the party trying to hook up with some real hot chicks that you haven't had sex with yet and if you can't get them to come back to your place tonight you always have the settler to fall back on because you know she is always down to get slammed.
Jon: Amanda is the settler tonight. She's such a slampig.
Ed: What's a settler?
Jon: Well if you can't bring any other slut back you gotta settle for her.