Screwed On 

Someone who is "all there" in a mental sense but not necessarily in a physical one.

Steven Hawking is totally screwed on man.

Screwed On by Simon Batt January 12, 2005

screwed on 

Opposite to someone who has a screw loose i.e a bit dopey.

You obviously had your head screwed on while you were tackling that piece of coursework.

screwed on by Simon Batt January 13, 2005