The word defecate was borrowed into the English language in the 1570s, and didn't necessarily have to do with excretion. The original definition was "to purify" (as in, when you defecate, you're purifying your body), and this comes from Latin defaecare, which had the same meaning. This is equivalent to de-, meaning "from", and the root faex, or "impurity" (so a purity comes from impurities). De-, which was sort of a catch-all prefix also meaning "down", "off", and "concerning", comes from a similar Proto-Indo-European root having some of those definitions, as well as "to". Faex (which is the direct etymon of feces, the pluralization of the word which was borrowed in the 1400s) has an intriguingly unknown history; it's proved very difficult to reconstruct or trace back. Faex populi is a phrase referring to the lowest classes of society and feculent means "full of impurities"; both of these terms are related through faex.
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