A British slang term describing a boisterous and usually drink-fuelled social state/attitude where persons "larging it" or "larging it up" are loud, noisy and playing around in a good humoured but macho way. Depending on the context, it can also be a way of squaring up against others without physical contact during real confrontations - a little bit like "mentally" sticking one's chest out as a sign of toughness, pretending to be tougher that one really is.
We were out on the town, drinking, larging it up, having a laugh.
When the bouncer grimaced at us and stared me in the eyes outside the nightclub, I went up to him, larging it up, saying, "What's your problem mate ?" He went "Boo !" to me, smiled, and said, "You're alright, you can come in !"