Green’s Dictionary of Slang

1. a session of drinking or taking drugs; thus attrib.

[1940		L. Pound ‘Guide to Variety’ AS XV:2 205/1: sesh. Session].
1995		I. Rankin Let It Bleed 47: He’s on for a sesh.
2000		N. Griffiths Grits 381: All a want-a do’s settle in for an all-day sesh.
2003		McGill Reed Dict. of N.Z. Sl. 182: sesh A session, especially smoking marijuana, but also gossiping. Teenage use late 1990s.
2012		(con. 1980s) I. Welsh Skagboys 123: A quick drink wi a couple of gadges fae Gillsland’s turned intae a bit ay a sesh.
2020		G. Armstrong Young Team 124: It’s a mad n sporadic sesh customer at the weekend.

2. any form of session.

2013

K. Sampson Killing Pool 176: And is if the sesh with Shakespeare wasn’t grim enough, the kart’s been keyed.