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