1550s, "member of a black-skinned race of Africa," from Spanish or Portuguese negro "black," from Latin nigrum (nominative niger) "black, dark, sable, dusky" (applied to the night sky, a storm, the complexion), figuratively "gloomy, unlucky, bad, wicked," according to de Vaan a word of unknown etymology; according to Watkins, perhaps from PIE *nekw-t- "night." The Latin word also was applied to the black peoples of Africa, but the usual terms were Aethiops and Afer.
As an adjective from 1590s. Use with a capital N- became general early 20c. (e.g. 1930 in "New York Times" stylebook) in reference to U.S. citizens of African descent, but because of its perceived association with white-imposed attitudes and roles the word was ousted late 1960s in this sense by Black (q.v.).
Professor Booker T. Washington, being politely interrogated ... as to whether negroes ought to be called 'negroes' or 'members of the colored race' has replied that it has long been his own practice to write and speak of members of his race as negroes, and when using the term 'negro' as a race designation to employ the capital 'N' [Harper's Weekly, June 2, 1906]
Meaning "African-American vernacular, the English language as spoken by U.S. blacks" is from 1704. French nègre is a 16c. borrowing from Spanish negro. Older English words were Moor and blackamoor. A Middle English word for "Ethiopian" (perhaps also "a negro" generally) was blewman "blue man."