spoil-sport (n.) Look up spoil-sport at Dictionary.com
1786, from verbal phrase (attested by 1711) in reference to one who "ruins" the "fun;" see spoil (v.) + sport (n.). Compare Chaucer's letgame "hinderer of pleasure" (late 14c.), from obsolete verb let (Middle English letten) "hinder, prevent, stop" (see let (n.)). Another old word for it was addle-plot "person who spoils any amusement" (see addle).