Ffeithio, v. to effectuate
Ffeithiol, a. effectuating; operative
Ffêl, a. subtile, fine; cunning, wily
Ffelaidd, a. of a subtile nature; sly
Ffelaig, n. perceptive source, a chief
Ffelder, n. subtilty, slyness
Ffelrwydd, n. craftiness
Ffelu, v. to act subtilely
Ffen, n. a flowing principle; air
Ffenestr, n. air hole; window, it is used figuratively for the vagina