Shest is an exaggerated way of saying 'Shit' (Shesty is the zesty way). It is mostly used after witnessing something shocking, but it can also be used to humor someone or when you have nothing to say(تسليك).
example 1:
person1: i got into an accident today and broke both my legs.
person2: Shest!
example 2:
person1: i failed my test person2: ...shesty
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.