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.