Why is the ninth month called September?
"notch on a bow," late 14c., of uncertain origin, probably from a Scandinavian source (cf. Swedish nock "notch"), but cf. also Low German nokk, Dutch nok "tip of a sail." Perhaps connected to nook.
"fit (an arrow) to a bowstring," 1510s, from nock (n.). Related: Nocked; nocking.