E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The dispersal of a large meeting in St. Peters Field, Manchester, by an armed force, August 16th, 1819. The assemblage consisted of operatives, and the question was parliamentary reform. The word, suggested by Hunt, is a parody upon what he absurdly called the bloody butchers of Waterloo.
It is a most exaggerated phrase. The massacre consisted of six persons accidentally killed by the rush of the crowd, when the military and some 400 special constables appeared on the field.