Spanked by mom and dad, drawing by Swatty
FM/m indicates corporal discipline for boys by two adults, male and female. It is a special case of the spanked by two scenario.
Traditionally, FM/m is a double spanking by both parents to discipline their unruly son. However, it can include any combination of two adults, such as one parent and her/his spouse, a headmaster and a mother, a grandparent and a parent. It is usually used as an additional form of embarrassment, where not only is the spankee spanked in front of another adult, but actually that other adult takes part in the spanking. A recurrent theme is that one adult holds the boy while the other spanks him, or both spank the boy at the same time.