Military in origin.

Pornographic material, traditionally magazines, left in a public restroom for all passing people to enjoy. Shitterporn gives the average man (or woman) visual stimulus while they masturbate. Finding shitterporn is usually considered a bonus, as most men are going to jack off while they take a shit anyway. Victoria's Secret catalogs have been observed as passable shitterporn.

Literally everyone, ever. Including the writer of this definition.