Green’s Dictionary of Slang

(Scot./Aus.) a psychologically unstable person, a lunatic.

1985		M. Munro Patter 44: maddy A maddy is a lunatic. To take a maddy is to lose your temper in a big way.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 41: The place was like a rabbit warren, with all the maddies humping each other, raping each other and attacking each other.