What it means
Means loud, rowdy, a bit gobshitey, and just one bad look away from aggro. You'd use it for a person, a group, or a whole pub when the vibe's gone from lively to someone might end up wearing their pint. It can also mean flashy or showy in that try-hard, look-at-me way.
Usage examples
"Place got proper lairy after the football, some lad’s squaring up over a spilt lager. We legged it to the chippy before it turned into WWE"
"It all kicked off when a few lairy lads started shouting at the bar staff over a spilled pint, proper embarrassing."
"Don't get lairy with the bouncer, mate, you're the one who can barely stand up."
"Pub was sound till that table in the corner got lairy and started gobbing off at everyone."
"He was getting well lairy outside the takeaway, chest out, shouting like he'd been elected mayor of the pavement."
Where it comes from
Lairy is loud, rowdy and a bit in your face, the mate who gets too boisterous after a few pints and starts mouthing off. It can also mean flashy or over the top. Probably from leery, wary, twisted round to mean the one others get wary of. Very British, the word for that edge between lively and trouble.