late 14c., "a foot race; an ancient measure of length," from Latin stadium "a measure of length; a course for foot-racers" (commonly one-eighth of a Roman mile or a little over 600 English feet; translated in early English Bibles by furlong), from Greek stadion "a measure of length; a race-course, a running track," especially the track at Olympia, which was one stadion in length. The meaning "running track," recorded in English from c. 1600, was extended to mean in modern-day context "large, open oval structure with tiers of seats for viewing sporting events" (1834).
"Originally the distance between successive stations of the shouters and runners employed to estimate distances" [Century Dictionary]. According to Barnhart, the Greek word might literally mean "fixed standard of length" (from stadios "firm, fixed," from PIE root *sta- "to stand"), or it may be from spadion, from span "to draw up, pull," with form influenced by stadios.
atheltic games and contests, 1590s, from sport (n.). Meaning "sports section of a newspaper" is 1913. As an adjective from 1897. Sports fan attested from 1921. Sports car attested by 1914; so called for its speed and power:
I have just returned from the south of France, passing through Lyons, where I visited the [Berliet] works with my car, and was shown the new model 25 h.p. "sports" car, and was so impressed with this that I immediately ordered one on my return to London. [letter in The Autocar, Jan. 7, 1914]
1570s of armies; from 1882 in field sports; by 1905 in the political sense (compare left wing). Right-winger is attested by 1919 in U.S. politics; 1895 in sports.