Green’s Dictionary of Slang

intoxicated by a drug, usu. MDMA.

1996		N. Blincoe ‘Ardwick Green’ in Champion Disco Biscuits (1997) 7: He was monged but he kept it together with a kind of feverish energy.
2000		Indep. on Sun. Culture 27 Feb. 14: Totally monged-out.
2000		N. Griffiths Grits 359: Weed so fuckin strong that ya monged afta arfa spliff.
2012		J. Fagan Panopticon (2013) 305: He’s totally monged on something.