Mental. Fucked in the head. Disturbed. Cool in a fucked up way. Has also gained currency as simply: foolish.
Orig. Rhyming Slang (UK), not necessarily only used by davids.
Back in the day when consumer electronics were very expensive, often needed maintainance/repair and we were poor, Radio Rental was a large British company which rented... TV sets. (Doh!)
Often shortened to just Radio.
Dude, put your cock away, we're on the fucking bus! Are you fucking radio, or what?
Vice City? Radio Rental mate.
Wow! Totally, like RRRRRRRRaaaaaaaaaaadioooooo RRRRRReeeental!
Shut it radio.