A furtling was a Victorian-era novelty in which one's hands or fingers were used to complement crude or erotic drawings.
A furtling was a drawing or photograph in which strategic holes had been cut over 'delicate areas' -- the buttocks, legs, crotch, and breast -- allowing one to put one's palm, fingers, or fingertips behind the cutout in a way to suggest various body parts. Fingertips became breasts, the fold of a knuckle became buttocks (or, in some racier examples, a vagina), and fingers became legs.
Most existing examples of these were fairly milquetoast. The most common are ink sketches used for advertisements, printed on small cards or beer mats with advertising copy and simple instructions for use printed on the back. However, there are also examples of erotic photographs with particularly naughty bits cut out; these are clearly still a novelty item, as a serious connoisseur would surely prefer the pictures in the unaltered state, but would also not be something that the local pub would distribute to customers.
While most of the controversy over furtlings (such as there was) focused on the ersatz lady-bits, a number of furtlings involved gentlemen mooning the viewer; these are still used today by Fentimans in their advertisements for Ginger Beer, Curiosity Cola and Victorian Lemonade. These are naturally accompanied by the slogan "Bottoms Up!".