The cap worn by the sword-bearer of the city of London, made hollow at the top like a custard.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Cushion Thumper * Custom-house GoodsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes