Green’s Dictionary of Slang

to manipulate a woman’s genitals.

1888–94		‘Walter’ My Secret Life (1966) IX 1738: A man on a set bending a little forward, and a woman standing in front of him — stink fingering of course.
1939		‘Justinian’ Americana Sexualis 37: to stinkfinger [...] Popular in adolescent and collegiate circles in which sexual activity is often limited to ‘petting’, i.e., pressing the female’s breasts and inserting the fingers into the vaginal orifice.