The practice during sex of stimulating one's partner's anus with one's finger. Enthusiasts draw attention to the wealth of nerve-endings in this region and hence the added pleasure that the practice may induce, especially at the point of orgasm. (See also anal beads, rimming, &c. &c.)

The word, like so many others of a naughty stripe, comes to us via the French, though its basis, the seventeenth-century postillion, was also a common English word. In the golden age of the horse-drawn carriage, the postillion was a chap who rode the leading near-side horse to help guide the team. We are to imagine, then, that in the mind of the coitally-engaged bottom-tickling Frenchman, the finger was the postillion to the penis's coachman. Oh, those French.

For guidance on the responsible enjoyment of postillionage, you might try Dr. Jack Morin's Anal Pleasure and Health or Dr. Alex Comfort's classic The Joy Of Sex.