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