This can mean in German "jewelerie" but in
English it can be slang for "old and boring person" or "very intolerant person". Basically anyone who says no to a lot of things can be called a "schmuck". This is not a compliment or a thing to say when you are trying to be polite.
Examples:
Me: hey teacher, may I borrow a ruler
Teacher: no.
Me: may I borrow a ruler from my friend?
Teacher: no.
Me: than from who
will I borrow a ruler?
Teacher: no one.
Me:
wow, you'
re such an old schmuck.