Sportsmen on the turf, who from experience and an acquaintance with the jockies, are supposed to be in the secret, that is, to know the true merits or powers of each horse; notwithstanding which it often happens that the knowing ones are taken in.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Knot * Knowledge BoxNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes