f/f is an abbreviation used in spanking art to signify an artwork or story that features a minor female spanker and a minor female spankee. In other words, a girl spanking a girl. It could be, for example, a girl spanking her playmate, another girl from her school, her sister, her cousin, et cetera. For younger girls, play spanking may come as part of a game of "house" or "school" where an older child will play the role of the spanker (mommy or principal or teacher) and spank the others when they are "naughty". Although this is roleplayed F/f, in reality it is f/f.
However, f/f not necessarily play but can also be a real punishment when an older sister is in left in charge of her younger sibling, or when a girl is spanked by her babysitter, a position often filled by teenage girls.
f/f is girl spanking art.