f/m is an abbreviation used in spanking art to signify an artwork or story that features a minor female spanker and a minor male spankee. In other words, a girl spanking a boy. It could be, for example, a girl spanking her playmate, a boy from her school, her brother, or her cousin.
In the case where a girl will play "house" or "school" with boys, the girl may take the role of a mommy, teacher or principal and spank her "son" or "schoolboy" for roleplayed misbehavior or poor work. Although this is roleplayed F/m, in reality it is f/m.
f/m is boy spanking art.