Etymology
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excrement (n.)
1530s, "waste discharged from the body," from Latin excrementum, from stem of excretus, past participle of excernere "to sift out, discharge," from ex "out" (see ex-) + cernere "sift, separate" (from PIE root *krei- "to sieve," thus "discriminate, distinguish"). Originally any bodily secretion, especially from the bowels; exclusive sense of "feces" is since mid-18c. Related: Excremental; excrementitious.
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excretion (n.)

c. 1600, "action of excreting;" 1620s, "that which is excreted," from French excrétion (16c.), from Latin excretionem (nominative excretio), noun of action from past-participle stem of excernere "to sift out, separate" (see excrement).

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*krei- 
Proto-Indo-European root meaning "to sieve," thus "discriminate, distinguish."

It forms all or part of: ascertain; certain; concern; concert; crime; criminal; crisis; critic; criterion; decree; diacritic; discern; disconcert; discreet; discriminate; endocrine; excrement; excrete; garble; hypocrisy; incertitude; recrement; recriminate; riddle (n.2) "coarse sieve;" secret; secretary.

It is the hypothetical source of/evidence for its existence is provided by: Greek krinein "to separate, decide, judge," krinesthai "to explain;" Latin cribrum "sieve," crimen "judgment, crime," cernere "to sift, distinguish, separate;" Old Irish criathar, Old Welsh cruitr "sieve;" Middle Irish crich "border, boundary;" Old English hriddel "sieve."
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cack (n.)
"excrement, act of voiding excrement," Old English (in cac-hus); as a verb, "to void excrement," mid-15c., from Latin cacare (see caca). Related: Cacked; cacking. Cack-handed (also cag-handed) "left-handed; awkward" is from 1854.
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copro- 

word-forming element indicating "dung, filth, excrement," before vowels copr-, from Latinized form of Greek kopros "dung," from PIE root *kekw- "excrement." Hence, coprology "study of obscene literature" (1856).

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caca (n.)
"excrement," c. 1870, slang, probably from Spanish or another language that uses it, ultimately from PIE root *kakka- "to defecate," which forms the base word for "excrement, to void excrement" in many Indo-European languages.
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scatology (n.)

"obscene literature," 1876, with -logy "treatise, study" + Greek skat-, stem of skōr (genitive skatos) "excrement," from PIE *sker- "excrement, dung" (source also of Latin stercus "dung"), on the notion of "to cut off;" see shear (v.), and compare shit (v.). Late 19c. dictionaries also give it a sense of "science of fossil excrement." Related: Scatological (1886).

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guano (n.)

c. 1600, from Spanish guano "dung, fertilizing excrement," especially of sea-birds on islands off Peru, from Quechua (Inca) huanu "dung."

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merdivorous (adj.)

"feeding upon dung," 1856, from Modern Latin, from Latin merda "dung, excrement" (see merde) + -vorous. Perhaps based in French merdivores (by 1830).

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poo (n.)
also pooh, baby-talk for "excrement," 1950s (see poop (n.2)).
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