Etymology
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occasion (v.)

mid-15c., occasionen, "to bring (something) about, be the cause of (something)," from occasion (n.), or else from Old French occasionner "to cause," from Medieval Latin occasionare, from Latin occasionem (see occasion (n.)). Related: Occasioned; occasioning.

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occasional (adj.)

late 14c., "occurring now and then," from occasion (n.) + -al (1) or from Old French ocasionel and directly from Medieval Latin occasionalis. Meaning "casual" is 1560s. Meaning "happening on or pertaining to a particular occasion" is from 1630s. Of furniture, etc., "adapted for use on special occasions," from 1749. Middle English also had occasionary "affording opportunity, favorable" (mid-15c.).

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occasionally (adv.)
c. 1400, "happening on some particular occasion," also "sometimes, happening as occasion presents itself, without regularity," from occasional + -ly (2).
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occident (n.)

late 14c., "western part" (of the heavens or the earth), from Old French occident (12c.) or directly from Latin occidentem (nominative occidens) "western sky, sunset, part of the sky in which the sun sets," noun use of adjective meaning "setting," from present participle of occidere "fall down, go down" (see occasion (n.)). As a geopolitical term, sometimes with a capital O, always somewhat imprecise.

With the definite article, the west; western countries; specifically, those countries lying to the west of Asia and of that part of eastern Europe now or formerly constituting in general European Turkey; Christendom. Various countries, as Russia, may be classed either in the Occident or in the Orient. [Century Dictionary, 1895]
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occidental (adj.)

c. 1400, "to, of, or in the west (of the sky or the earth)," from Old French occidental (14c.) and directly from Latin occidentalis "western," from occidentem (see occident). Meaning "of, pertaining to, or characteristic of the western regions of the earth (especially Western Europe and its derivative civilizations in the western hemisphere" (opposed to oriental), 1550s. As a capitalized noun meaning "a Western person" (opposed to Oriental) it is attested from 1823. Related: Occidentalism; occidentalist.

Those who inhabit (to us) the western regions of the world, and to express whom the English language wants a word, the opposite of Orientals; though word-coining be much condemned, I will venture to employ Occidentalsas substantive and say, (etc.) ["The Bee," 1823] 
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occipital (adj.)

"of, on, or in the back of the head," 1540s, from French occipital, from Medieval Latin occipitalis, from Latin occiput (genitive occipitis) "back of the skull," from assimilated form of ob "in the way of, against," here with a sense of "in back of" (see ob-) + caput "head" (from PIE root *kaput- "head"). As a noun, "the occipital bone," from 1758. Middle English had occiput (n.) "back of the head" and occipiciale (n.) "occipital bone."

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Occitan (n.)

"Old or modern Provençal; langue d'Oc," 1940, also "the northern variant of modern Provençal;" from French oc,  the word used south of the Loire for "yes" (see Languedoc).

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occlude (v.)

"to shut up or stop up so as to prevent anything from passing through," 1590s, from Latin occludere (past participle occlusus) "shut up, close up," from assimilated form of ob "in front of, against" (see ob-) + claudere "to shut, close" (see close (v.)). Of teeth, "come in contact with another tooth," 1888. Related: Occluded; occluding.

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occlusion (n.)

"act or fact of being stopped up," 1640s, from Medieval Latin occlusionem (nominative occlusio), noun of action from past-participle stem of Latin occludere (see occlude). Dentistry sense "position of the two sets of teeth relative to each other when the mouth is closed" is from 1880.

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occlusive (adj.)

"serving to close, having the function of closing," 1867, from Latin occlus-, past-participle stem of occludere (see occlude) + -ive.

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