The term probably gets its origins from the pub days in the UK where folks would drink milds, bitters, Scottish “light” or 60 Shillings – these styles are all typically considered session-strength at <4% ABV.

Where does session beer come from?

It allowed English factory workers to drink on the job during two daily sessions. The workers would have low-ABV beers so they weren't stumbling back onto the job, hence the terminology “session beer.” Others say session simply refers to a long stay at the bar.

Who owns session beer?

Session Lager | Full Sail Brewing Company | BeerAdvocate.