Hinged seems like a great candidate for a slang term. Like, that guy's totally hinged. Logically, I would use it for people that seem to have their head on straight, or things that make sense.
I was intrigued by this so I had a look in the good ol' dictionary, and whelmed is a word... Although it's meaning is closer to overwhelmed than it is to, say, nonplussed as you might assume. The "over" has been added for emphasis rather than to change the meaning, and I assume underwhelmed came later.