Feces

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late Middle English: from Latin, plural of faex ‘dregs’.

wiktionary

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From Latin faecēs, nominative plural of faex(“residue, dregs”).

etymonline

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feces (n.)

also faeces, c. 1400, "dregs," from Latin faeces "sediment, dregs," plural of faex (genitive faecis) "grounds, sediment, wine-lees, dregs," which is of unknown origin. Specific sense of "human excrement" is from 1630s in English but is not found in classical Latin. Hence Latin faex populi "the dregs of the people; the lowest class of society."