One possibility of the origin is that it is a version of the Old English cocc which means "the little," and was sometimes placed after the name of a leader or chieftain as a term of endearment. Surnames such as Wilcox, Willcocks and Willcox are examples of this practice: all are composed of the name William and the archaic word cocc, coming together to mean "little William." The suggestion is that only the element -cox may have endured as a surname for some families.

Another opinion is that the name is derived from the Old English cock, which means a "heap" or "mound," and was a topographic name for a man living near any heap, hill or other bundle. Names like Haycock or Haycox come from such practice, meaning from "the hay mounds" or "the hay fields." Again, the element -cox may have only been carried on in some families.

The third possibility is that it comes from the Welsh coch, meaning "red." In this opinion, the word could have either been applied to a man with red hair, calling him in essence "the Red," or else served as a topographic name for someone living near the rudy-hued hills found in Wales, implying that the man is "from the red hills." In Cornwall, the surnames Cock and Couch (pronounced 'cooch') also derive from Cornish cogh "red, scarlet". As a Cornish surname, Cock can also derive from 'cok', "fishing boat", the Cornish surname "Cocking" being the diminutive form 'cokyn', "small fishing boat". In these cases, the surname is likely to derive from occupation.

The English word cock is derived from the French coq, and a fourth possibility, though a very limited one (as the surname had already been established in many parts of Great Britain), is that the surname came about as a nickname from the French language.

Noticeably similar surnames include Cock, Cocks, Coxe, Coxen and Coxon. There is no evidence beyond similar spellings and phonetics that these surnames are related. Given that the origins of the Cox surname are uncertain, it is possible that these names developed as spelling variations, or that each of these names has an origin in a separate word and language.