A smoke sesh is the time you spend smoking one round of weed. It refers to both the length of time, the place you're smoking, and the experience itself. You can compare it to how we use the words "meal" or "appointment". You could say "this weed will last me five sessions," referring to the length of time it will last based on how much cannabis on average you smoke at one time. You could say "I need to bring this pipe to my next smoke sesh," meaning the physical space you consume. And you could say "I enhanced my sesh by listening to Radiohead," referring to the abstract experience of smoking weed itself.
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