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.

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.