• noun Brit. Informal a young man who behaves offensively as a result of excessive drinking
The term lager lout was coined in the 90s by the tabloid press . A lager lout is an uncouth, young male whose anti social behaviour is as a result of excessive drinking - fueled by the cheap beer readily available in supermarkets. LLs are also associated with the rise of football hooliganism.
The term is now generally used to refer to the happy but rowdy drunks that you find in most city centres on Saturday night.