Shest is an exaggerated way of saying 'Shit' (Shesty is the zestyway). 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