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To draw in one’s horns; to retract an assertion through fear: metaphor borrowed from a snail, who on the apprehension of danger, draws in his horns, and retires to his shell.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hopper-arsed * Horn ColicNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes
Buy a modern reprint of this book from Amazon CA; US;
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.