The word "Transponster" was a word "invented" by Rachel during The Contest for the Monica and Rachel's apartment.
Transponster is not actually a word. It was screamed by Rachel during The Contest. It is a joke presumably written by the writers following the idea that nobody knows what Chandler does for a living.
When Ross asks what Chandler does for a living, Chandler and Joey are clearly excited, as they know they are going to win. Rachels says the incorrect answer, and Monica screams, "That's not even a word!"