Slang meaning of "pod"

From Seinfeld, season 2 episode 5 ("The Apartment"):

JERRY: Let me explain something to you.. You see, you're not normal. You're a great guy, I love you, but you're a pod. I, on the other hand, am a human being. I sometimes feel awkward, uncomfortable, even inhibited in certain situations with the other human beings. You wouldn't understand.

KRAMER: Because I'm a pod?

What's the meaning of "pod" here? Any reference for this / usage in this sense other than in Seinfeld?

I'll answer my own question based on the link from user "Kyle" in a comment to the original question:

Based on this thread, discussing this exact Seinfeld episode, a "pod" here "is a reference to the pod people from the science-fiction book (and movie) Invasion of the Body Snatchers. As Wikipedia explains, the pods were a race of extraterrestrial parasites that "floated in space like spores," occasionally landing on planets and "spawning emotionless replicas." Thus, a pod person, or "pod," looks like any other human being, but is actually an emotionless alien".