Green’s Dictionary of Slang

lean adj.

(UK drugs) intoxicated by cannabis.

2001		Da Smokehouse Marijuana Gloss. 🌐 leaned – stoned (usually describing the first stages of stonedness).
2005		DJ Cameo 1Xtra [BBC radio] Big shout to everyone sitting around getting mash-up lean.
2016		Skepta ‘Numbers’ 🎵 Cause everyday manaman a get lean.

In phrases

sit on lean (v.)

to be intoxicated by drugs.

1996

UGK ‘3 in the Mornin’’ 🎵 Done flagged me down for some more of that brown / To go with that green, now they sittin’ on lean.