Green’s Dictionary of Slang

donigan n.

a privy.

1914		Jackson & Hellyer Vocab. Criminal Sl. 41: I must drop into the hotel donegan (lavatory) and fire (take a hypodermic injection), for I feel my habit coming on.
1931		Irwin Amer. Tramp and Und. Sl. 64: Donegan.–A toilet or wash-room, The English lowslang equivalent is ‘dunnyken’ or ‘dunnaken,’ originally ‘dannaken,’ a water-closet.
1949		Monteleone Criminal Sl. (rev. edn) 71: donigan A toilet; a privy; a rest room donigan slicker One who robs his victim in a rest room donigan work robbery in a rest room.