Cross between love-rat and bastard. Term means anyone who is coniving, and who makes sure their partner is the last to know of their activities.
Non-puristically, It is used as a general insult to anyone.
Get away from my truck, you goddam lil' ratbastard!
A commitmentphobic who carelessly or negligently mistreats daters as a direct result of said commitmentphobia. Ratbastardly actions center around lying, cheating, ending dating relationships irresponsibly.
After five months together he just stopped calling without an explanation. What a ratbastard!!