M25, n. — Green’s Dictionary of Slang

M25 n.

[the initial letters + the UK’s M25 motorway, an orbital route round London, which played a major part in the siting of the early ecstasy-fuelled raves]

(drugs) MDMA.

2002		DRUG-ARM Aus. 🌐 Slang Terms: m25 MDMA (Ecstasy).
2002		London NHS Trust Community Drug Service ‘The Facts About Ecstasy’ In the Limelight 🌐 What is ecstasy? [...] It is also called E, XTC, Adam, Love Doves, New Yorkers, M25’s, Disco Burgers, Dennis The Menace to name a few.