To give law to a hare; a sporting term, signifying to give the animal a chance of escaping, by not setting on the dogs till the hare is at some distance; it is also more figuratively used for giving any one a chance of succeeding in a scheme or project.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
launch * lawful blanketNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes