postilion, v. — Green’s Dictionary of Slang

postilion v.

[SE postilion, a swift messenger; thus the practice makes one ‘come v.¹ faster’]

to insert and manipulate a finger in the anus of a sexual partner as a means of increasing sexual excitement.

1880		Power of Mesmerism [ebook] My left hand passed round his loins, caressing his fine prick and balls as it worked in and out of her beautiful brown bumhole, whilst my right forefinger postilioned him behind.
1888		Tableaux Vivants (translation) xi 95: The fair houri was postillioning me [OED].
1890–1904		Farmer & Henley Sl. and Its Analogues.
1969		G. Legman Oragenitalism i 90: Postillioning can best be done by the middle finger.