A guide post, hand or finger post by the road side for directing travellers: compared to a parson, because, like him, it sets people in the right way. See Guide Post. He that would have luck in horse-flesh, must kiss a parson’s wife.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Park Pailing * Parson’s JourneymanNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes