Etymology
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omicron (n.)

15th letter of the Greek alphabet, c. 1400, literally "small 'o,' " from o + Greek (s)mikros "small" (see micro-). So called because the vowel was "short" in ancient Greek. Compare omega.

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Mexico 

republic lying to the south of the U.S., from Spanish, from Nahuatl (Aztecan) mexihco, which originally referred to the Valley of Mexico around present-day Mexico City. It became the name of the nation (formerly New Spain) upon independence from Spain in 1821.

The etymology of this is opaque. Because of the difference in vowel length, it cannot be derived from ME-TL 'maguey.' The sequence XIH also differs in vowel length from XIC-TLI 'navel,' which has been proposed as a component element. The final element is locative -C(O). [Kartunnen]
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gu- 
because g- followed by some vowels in English usually has a "soft" pronunciation, a silent -u- sometimes was inserted between the g- and the vowel in Middle English to signal hardness, especially in words from French; but this was not done with many Scandinavian words where hard "g" precedes a vowel (gear, get, give, etc.). Germanic -w- generally became -gu- in words borrowed into Romance languages, but Old North French preserved the Frankish -w-, and English sometimes borrowed both forms, hence guarantee/warranty, guard/ward, etc.
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sack (n.4)

"sherry," 1530s, an alteration of French (vin) sec "dry (wine)," from Latin siccus "dry" (see siccative). Originally of strong, light-colored wine from Spain and the Canaries. OED notes that the vowel is "not a normal development from the original 'seck.' "

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deca- 
before a vowel, dec-, word-forming element meaning "ten," from Latinized combining form of Greek deka "ten" (from PIE root *dekm- "ten"). In the metric system, "multiplied by ten;" while deci- means "divided by ten."
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indelible (adj.)
1520s, from Latin indelebilis "indelible, imperishable," from in- "not, opposite of, without" (see in- (1)) + delebilis "able to be destroyed," from delere "destroy, blot out" (see delete). Vowel change from -e- to -i- in English is late 17c. Related: Indelibly.
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collide (v.)

"to strike together forcibly," 1620s, from Latin collidere "strike together," from assimilated form of com "with, together" (see com-) + laedere "to strike, injure by striking," which is of unknown origin. For Latin vowel change, see acquisition. Related: Collided; colliding.

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cyclo- 

before a vowel, cycl-, word-forming element in technical terms meaning "circle, ring, rotation," from Latinized form of Greek kyklos "circle, wheel, ring" (from PIE root *kwel- (1) "revolve, move round"). In organic chemistry it is used in forming chemical names of cyclic compounds.

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triennial (adj.)
1630s, "lasting three years;" 1640s, "occurring every three years," with -al (1) + Latin triennium "three-year period," from tri- "three" (see tri-) + annus "year" (see annual (adj.)). For vowel change, see biennial. As a noun, 1630s. Related: Triennially.
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precentor (n.)

"a leader or director of a church choir or congregation in singing," 1610s, from Late Latin praecentor "a leader in singing," from Latin praecantare "to sing before," from prae "before" (see pre-) + canere "to sing" (from PIE root *kan- "to sing"). For change of vowel, see biennial.

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