stimming from the group of people who originally used this term to spread awareness of the people who have autism being quite obnoxious and toxic this refers to a human who is "acing" the embodiment of a low functioning autistic individual who cannot function in society or is acting like a person with down-syndrome.
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.