c.1300, from Old French espoillier "to strip, plunder," from Latin spoliare "to strip of clothing, rob," from spolium "armor stripped from an enemy, booty;" originally "skin stripped from a killed animal," from PIE *spol-yo-, perhaps from root *spel- "to split, to break off" (cf. Greek aspalon "skin, hide," spolas "flayed skin;" Lithuanian spaliai "shives of flax;" Old Church Slavonic rasplatiti "to cleave, split;" Middle Low German spalden, Old High German spaltan "to split;" Sanskrit sphatayati "splits").
Sense of "to 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 "to go bad" 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. Spoil-sport attested from 1801.