Someone who is "all there" in a mental sense but not necessarily in a physical one.
Steven Hawking is totally screwed on man.
מאת Simon Batt 12 בינואר, 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.
מאת Simon Batt 13 בינואר, 2005