They are about to get sorted into their houses at hogwarts. And Ron says "Mental that one. I'm telling you." But what's the meaning of mental here? Can anyone help me?
It means mentally ill or, in other words, crazy. Crazy enough to be sent to a mental hospital. This is primarily a British expression. Have you heard of Da Ali G Show? He uses that expression sometimes.
Here's his famous address to graduating students during the 2004 commencement ceremony at Harvard: https://www.youtube.com/watch?v=GUCy75CA3Aw#t=11m4s
Let's talk about the finances of all that knowledge that's been dropped on you the last few years. It costs $38,000 a year to go to Harvard. Now, I don't know how you lot has earned that. Probably from you. You is earned every penny. But most of you has got that cash from your parents. All you fathers out there, you is made some choices. With that money, you could've bought a top of the range Lexus, but instead you chose to invest in your kids' future. Is you mental?
So, when somebody says Are you mental?, what they mean is Are you out of your goddamn mind?