Merriam-Webster Collegiate Dictionary - nock

 I. noun  Etymology: Middle English ~e notched tip on the end of a bow; akin to Middle Dutch ~e summit  Date: 14th century  1. one of the notches cut in either of two tips of horn fastened on the ends of a bow or in the bow itself for holding the string  2.  a. the part of an arrow having a notch for the bowstring  b. the notch itself  II. transitive verb  Date: 14th century  1. to make a ~ in (a bow or arrow)  2. to fit (an arrow) against the bowstring