A fist is a hand when the fingers are bent in towards the palm and held there tightly. To make or clench a fist is to fold the fingers tightly into the center of the palm and then to clamp the thumb over the middle phalanges; in contrast to this "closed" fist, one keeps the fist "open" by holding the thumb against the side of the index finger. One uses the closed fist to punch the lower phalanges against a surface, or to pound with the little-finger side of the hand's heel; one uses the open fist to knock with the middle knuckle of the middle finger.
