spoil-sport n.

c. 1300, "to strip (someone) of clothes, strip a slain enemy," from Old French espillier "to strip, plunder, pillage," from Latin spoliare "to strip, uncover, lay bare; strip of clothing, rob, plunder, pillage," from spolia, plural of spolium "arms taken from an enemy, booty;" originally "hide, skin stripped from a killed animal," from Proto-Italic *spolio- "skin, hide," from PIE *spol-yo-, probably from a root *spel- (1) "to split, to break off" (see spill (v.)) on the notion of "what is split off."

From late 14c. in English as "strip with violence, rob, pillage, plunder, dispossess; impoverish with excessive taxation." Used c. 1400 as the verb to describe Christ's harrowing of Hell. Sense of "destroy, ruin, damage so as to render useless" is from 1560s; that of "to over-indulge" (a child, etc.) is from 1640s (implied in spoiled). Intransitive sense of "become tainted, go bad, lose freshness" is from 1690s. To be spoiling for (a fight, etc.) is from 1865, from notion that one will "spoil" if he doesn't get it.

early 15c., "pleasant pastime," shortening of disport "activity that offers amusement or relaxation; entertainment, fun" (c. 1300), also "a pastime or game; flirtation; pleasure taken in such activity" (late 14c.), from Anglo-French disport, Old French desport, deport "pleasure, enjoyment, delight; solace, consolation; favor, privilege," related to desporter, deporter "to divert, amuse, please, play" (see sport (v.)), also compare disport (n.).

Original sense preserved in phrases such as in sport "in jest" (mid-15c.). Meaning "game involving physical exercise" first recorded 1520s. Sense of "stylish man" is from 1861, American English, probably because they lived by gambling and betting on races. Meaning "good fellow" is attested from 1881 (as in be a sport, 1913). Sport as a familiar form of address to a man is from 1935, Australian English. The sport of kings was originally (1660s) war-making. Other, lost senses of Middle English disport were "consolation, solace; a source of comfort."