Nock, n. & v.
--n.
1 a notch at either end of a bow for holding the string.
2 a a notch at the butt-end of an arrow for receiving the bowstring. b a notched piece of horn serving this purpose.
--v.tr. set (an arrow) on the string.
ME, perh. = nock forward upper corner of some sails, f. MDu. nocke