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Licensed itinerant retailers of different commodities, called also pedlars; likewise the sellers of news-papers. Hawking; an effort to spit up the thick phlegm, called OYSTERS: whence it is wit upon record, to ask the person so doing whether he has a licence; a punning allusion to the Act of hawkers and pedlars.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hawk * To Hazel GildNathan 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.