Frean, though usually from the Scandinavian name fraena, is sometimes for Fr.
frenum fre·num (frē'nəm)
n. pl. fre·nums or fre·na (-nə)
A membranous fold of skin or mucous membrane that supports or restricts the movement of a part or organ, such as the small band of tissue connecting the tongue to the floor of the mouth.
An anatomical structure resembling such a fold.