A schnook is a person who is easily cheated or taken advantage of.
Calling someone a schnook is not very nice. It means they are not very smart or are easily tricked.
Examples:
He's such a schnook, he believed everything the salesman said.
Don't be a schnook and fall for that trick.
The schnook didn't realize he was being tricked.
I felt like a schnook when I realized I paid too much for the broken toy.
He's the biggest schnook in the whole town.
The poor schnook didn't know what hit him.