Someone who is "all there" in a mental sense but not necessarily in a physical one.
Steven Hawking is totally screwed on man.
ayon kay Simon Batt ika-12 ng Enero, 2005
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.
ayon kay Simon Batt ika-13 ng Enero, 2005