The F-finger

The joys of parenting are pockmarked by milestone moments of infinite discomfort felt by the parent. Like this for example. My daughter, all of 7 this June, comes home from her coed school and asks me to explain what the F-finger is. I told her such a finger does not exist. She then holds up her middle finger to me with all the panache and attitude that is needed when resorting to the ‘up yours’. A boy in her class showed her the finger and called it the F-finger. I told her I’ll get back to her. Any suggestions?