Green’s Dictionary of Slang

to insert and manipulate a finger in the anus of a sexual partner as a means of increasing sexual excitement, thus n. postillion, digital stimulation of the anus with a finger or a dildo.

1873		Romance of Lust (1968) II 202: ‘You can gamahuche her and Lizzie can look behind, witness the glorious sight, and act postilion to my botton-hole’ [ibid.] 205: I was to fuck Miss Frankland’s cunt, and postillion her smaller orifice, Lizzy was to postilllion Mary.
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.
1883		Pretty Women of Paris 127: we have been told that she handles in a masterly manner a cunning shaft of ivory, with with she figures a pederastic attack during the act of copulation. Let all those who require a postilion rush to her at once [Simes:DLSS].
1888		Tableaux Vivants (translation) xi 95: The fair houri was postillioning me [OED].
1890–1904		Farmer & Henley Sl. and Its Analogues.
c.1900		‘Ramrod’ My Lustful Adventures (1996) 29: Occasionally she titillated my bollocks, or ‘postillioned’ my anus as she called it.
1969		G. Legman Oragenitalism i 90: Postillioning can best be done by the middle finger.