When the word chaos was borrowed into English from Old French in the late 1300s, it specifically referred to Tartarus, the prison of the underworld in Greek mythology. As such a niche term, it didn't get much usage until the seventeenth century, when a resurgence in interest in Greek and Roman culture led to more widespread adaptation of the word, and, subsequently, change to what it is today. The Old French word chaos comes from the Latin word chaos, which comes from Ancient Greek khaos, which still had the same meaning. That in turn has an uncertain etymology, but some linguists have found links to the words khaskho, meaning "gape", and khaino, "to yawn". If that theory is correct, and the word is in reference to the size of the hole of Tartarus, it can be derived back to Proto-Indo-European ghieh, "to be wide open".
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