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A seal. Onion hunters, a class of young thieves who are on the look out for gentlemen who wear their seals suspended on a ribbon, which they cut, and thus secure the seals or other trinkets suspended to the watch.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
One of Us * Open ArseNathan 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.