In this particular episode, Jerry dated a woman who had never laughed at his jokes and only replyed "That's funny." He later met her beautiful room-mate and was sinfully attracted to her (ironically laughing to all his jokes).
Jerry then tells this to George and both of them plans the "Switch" which has never been done before. Amazingly, George finally came up with a scheme to say this to the girlfriend: " I'm having a 'manaja tua' with your room-mate" I don't know the correct spelling of it. After saying it to her, Jerry seemed to have both of them instead, much to George's envy.
The episode after that, George mentioned the word again to his girl with the velvet couch and a male room-mate and was "attacked" by him.
I don't think this 'manaja tua' means something good but I would really like to know, if not I'll never understand what the joke is all about. And being Seinfeld fans I'm sure you know how that feels?!