To damage or ruin something, or to treat someone with excessive indulgence.
From Old French espoillier 'to strip, plunder,' from Latin spoliare 'to strip of clothing, rob.' The sense of 'ruin by indulgence' developed in the 16th century, while 'decay' (as in food spoiling) emerged in the 18th century.
The word 'spoil' carries the fascinating evolution from literal plundering in warfare to the metaphorical 'plundering' of someone's character through overindulgence. Food that spoils is essentially being 'plundered' by bacteria!
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