The primary sense is a head or leader. Gaelic, ceann; Welsh, cun and cwn, a head, a leader. Saxon, cyng, and nearly the same in all the Teutonic dialects.

Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.

KING. A very common sobriquet in all ages and countries. Classical antiquity affords us the names of Basilius, Archias, Regulus, Caesarius, &c., borne by people wlio, as Camden quaintlj' remarks, "were neither kings, dukes, nor Ca?sars." There are plenty of Lerois in France, and Kcenigs in Germany, who are of no royal descent, and it is only within a few generations that the ' Kings ' of England have emerged from a plebeian grade. The name may very probably have originated in those popular medieval pastimes in which Kings of the Bean — of May — of Cockneys — of Misrule held temporary sway. For their functions see Brand's Pop. Antiq. edit., 1812.

Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.