A parson: so called, because he points out a way to others which he never goes himself. Like the finger post, he points out a way he has never been, and probably will never go, i.e. the way to heaven.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Finger In Eye * FinishNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes