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Irish robbers, or outlaws, who in the time of Oliver Cromwell were armed with short weapons, called in Irish RAPIERS, used for ripping persons up.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
To Rap * RapperNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes
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Francis Grose was independently wealthy, having inherited money from his father, a jeweller. Finding himself overspending, he published a number of books; his Provincial Glossary seems to have been the starting-point for the Vulgar Tongue reproduced here.