Proper noun
Peterloo
(historical) The events of 16 August 1819, when cavalry charged into a large crowd that had gathered in St Peter's Field, Manchester, England, to demand the reform of parliamentary representation, killing fifteen and injuring hundreds more.
He had a pop at, among others, Lord Eldon who as lord chancellor, played a key role in drafting Acts to gag newspapers, prevent public meetings, and restrict freedom of speech after Peterloo. Source: Internet
One such gathering ended with the Peterloo Massacre of 16 August 1819. Source: Internet