Green’s Dictionary of Slang

(US drugs) to experience MDMA; usu. as x-ing.

1986		Eble Campus Sl. Oct. 6: x-out – get high on a hallucinogenic drug (from ecstacy): She x-ed out at the party.
2001		ONDCP Street Terms 23: X-ing — Methylenedioxymethamphetamine (MDMA).