A douchebag that sleeps with his ex girlfriends cousin and doesn't think anything of it. Ratfinks usually have no job, no car, and sleep on peoples couches for 4 months at a time. They also tend to have wide nostrals, a pointy nose, and a lack of chin. Beware; ratfinks love to cheat. Stay away from any ratfinks you know...they may be contagious.
Eric L: What, I never did that. (knowing he's lying)
Corina: Fuck you, you fucking ratfink!!!