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Pistols. Popshop: a pawnbroker’s shop. To pop; to pawn: also to shoot. I popped my tatler; I pawned my watch. I popt the cull; I shot the man. His means are two pops and a galloper; that is, he is a highwayman.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Pope’s Nose * PoplersNathan 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.