Robert: I really wish I can describe how I felt hanging out with you... not yesterday, but the day before yesterday.
Mia: You mean befesterday?
Robert: Wow, what a genius, concise word that the English Language does not have yet...
A person pretending to be something they aren't, or an impostor. This is inferred because of the old alien movies where alien pods appear on earth and the "pod people" dispose of the humans and slowly reproduce the bodies, pretending to be humans.
The pod person was pretending to be my father, but I knew that he was an alien when he acted differently than my father ever would have.