A "stuck up" person is someone who:

• thinks that they are better than other people
• thinks that other people are less important than them
• dislikes people who are poorer, less popular, or less powerful than themselves

It's 

Here are some example sentences:

I can't stand her. She acts like a stuck up little princess.

I guess I should go out drinking with them sometime. I don't want to be seen as rude or stuck up, you know?