I don't know which episode this line is from, but I recall it's from that episode where Jerry and his friends "intervene" on a fellow comic who on drugs. This was also the episode where George dates that piano player and breaks up "pre-emptively" with her.
The funniest line from that episode was given by some old guy who is in the Polar Bear's club. The old guy was talking to Elaine during Jerry's intervention:
"The male kangaroo doesn't have a pouch, only the female has it. So the male has pouch envy. Why should she have this huge pouch and I have nothing? I have things to carry too. At least give me a pocket."