craynade

Noun: A cray cray (crazy) AND volatile person who will cause substantial collateral damage when they go off - a crazy-grenade.

Variation: A craynade with the pin out.

Dude 1: Ol' loony Pauley just rolled up in here.
Dude 2: That guy's a craynade - let's get out of here before someone pulls his pin and the shrapnel starts flying.