Psst is defined as a way to attract someone's attention, usually when you are trying to be quiet about getting attention.
If you want to get your friend's attention in a movie theater after the movie has already started, this is an example of when you might whisper "psst."
Informal used to attract someone's attention, usually in an unobtrusive way
Origin of psstechoic of a hiss
Used to capture someone's attention inconspicuously.
Origin of psstImitative