The British-English colloquial term lager lout designates a young man who behaves in an unpleasant or aggressive manner as a result of drinking (typically lager) excessively.

This term is first recorded in Beware the Barmaid’s Smile!: The New Vulgarity in Our Pub Culture (London: Pleasure Tendency, 1987), a pamphlet by ‘Chris Thompson’, pseudonym of the Scottish architect and political activist George Williamson (1939-2007). In this pamphlet, the author, who had worked for brewery companies, mostly on pub design, demanded that the evolution of the pub be controlled by the customers and not by the breweries, calling for militant opposition to the remorseless corporatisation of pubs and the brewing industry:

When the lager lout says that beer is an old man’s drink, the reply is to ask if they have ever thought of growing up.

The opposition between lager and beer as “an old man’s drink” in the above-quoted passage from George Williamson’s pamphlet was explained as follows in a report from Coventry, England, by the Associated Press, published in many U.S. newspapers on Thursday 6^th October 1988—for example in The Muncie Evening Press (Muncie, Indiana):

Lager is a pale, American-style beer favored by the young as opposed to the dark, traditional “bitter” English beer.

This is a second extract from this Associated Press report:

Coventry, England (AP)—The government says it will ban public drinking in eight English cities next year in an experimental crackdown on so-called “lager louts”—rowdy youths who get drunk on American-style beer.
[…]
The measure follows a wave of summer violence in Britain’s suburbs and rural towns by young men, most of them between the ages of 18 and 26, who gather on weekends and drink on the streets or in local pubs.
They have been blamed for brawls, vandalism and attacks on unarmed police called to disperse the drunken, rowdy crowds.
Police estimate dozens of officers have been injured in confrontations nationwide with the so-called “lager louts.”

The earliest occurrence of lager lout that I have found is from the Derby Evening Telegraph (Derby, Derbyshire, England) of Wednesday 28^th September 1988:

Lager lout kicked man senseless in shop row

Lout Shane Askew has been put away for 30 months for kicking a man senseless outside an Ilkeston fish and chip shop.
Derby Crown Court heard yesterday how Askew had drunk 10 pints of lager before attacking 25-year-old Shaun Fogg outside the town’s Fish Inn.
The victim was left almost unconscious when Askew (18) knocked him to the ground and kicked him.
He appeared in court on the same day the Government announced a tough new advertising clamp down to curb street violence by drunken youths.

The second-earliest occurrence of lager lout that I have found is from Soccer Diary, by Mike Rowbottom, published in The Guardian (London and Manchester, England) of Saturday 1^st October 1988:

And here he is. Unearthed in a recent Museum of London excavation near Liverpool Street station, the ancient forbear of today’s lager-lout soccer yobbo. Museum officials are still labouring under the impression that the chappie on their mediaeval tile is the legendary Green Man. More enlightened scholars have grasped the truth of the matter, and debate now centres on whether this is a southern fan or a migrant from the north who failed to make it back to King’s Cross.

The term lager lout has been associated with hooligan—as in the following from the column Across the Pond, by Suzanne Cassidy, published in the Lancaster New Era (Lancaster, Pennsylvania) of Saturday 4^th March 1989:

English football—which is not what Americans would call football at all, but soccer—is known to attract some pretty heavy fans. They’re called “lager louts” or “football hooligans” or more simply “yobbos.”
They’re usually young white men prone to drinking huge quantities of strong lager and equally prone to racism, fascism and sexism. Their penchant for violence is renowned.