Etymology
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obtund (v.)

c. 1400, obtunden, (transitive) "to render dead, make dull, blunt, deaden," used occasionally in English, especially in medical jargon; from Latin obtundere "to blunt, make dull, weaken, exhaust," literally "to beat against" (see obtuse). Related: Obtundation; obtunded.

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obtuse (adj.)

early 15c., "dull, blunted, not sharp," from Latin obtusus "blunted, dull," also used figuratively, past participle of obtundere "to beat against, make dull," from ob "in front of; against" (see ob-) + tundere "to beat," from PIE *(s)tud-e- "to beat, strike, push, thrust," from root *(s)teu- "to push, stick, knock, beat" (source also of Latin tudes "hammer," Sanskrit tudati "he thrusts"). Sense of "stupid, not acutely sensitive or perceptive" is by c. 1500. In geometry, in reference to a plane angle greater than a right angle," 1560s. Related: Obtusely; obtuseness.

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obverse (adj.)

"turned toward the observer, frontal," 1650s, from Latin obversus "turned against, directed toward," past participle of obvertere "to turn toward or against," from ob "toward" (see ob-) + vertere "to turn" (from PIE root *wer- (2) "to turn, bend"). According to OED, not in common use until the end of the 18th century. The noun, in reference to coins, medals, etc., "side on which the head or principal design is struck" (opposite of reverse), is attested from 1650s. Related: Obversely.

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obviate (v.)

1590s, "to meet and dispose of, clear (something) out of the way," from Late Latin obviatus, past participle of obviare "act contrary to, go against," from Latin obvius "that is in the way, that moves against," from obviam (adv.) "in the way," from ob "in front of, against" (see ob-) + viam, accusative of via "way" (see via). Related: Obviated; obviating.

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obviation (n.)

c. 1400, obviacioun "encounter, contact; exposure," from Medieval Latin obviationem (nominative obviatio), noun of action from past-participle stem of obviare "act contrary to, go against" (see obviate).

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obvious (adj.)

1580s, "frequently met with" (a sense now obsolete), from Latin obvius "that is in the way, presenting itself readily, open, exposed, commonplace," from obviam (adv.) "in the way," from ob "in front of, against" (see ob-) + viam, accusative of via "way" (from PIE root *wegh- "to go, move, transport in a vehicle"). Meaning "plain to see, evident" is recorded from 1630s. Related: Obviously; obviousness.

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oc- 
assimilated form of ob- before -c-.
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ocarina (n.)

simple musical instrument with a terra-cotta body, a mouthpiece, and finger-holes, 1877, from Italian ocarina, diminutive of oca "goose" (so called for its shape), from Vulgar Latin *auca, from Latin avicula "small bird," diminutive of avis "bird" (from PIE root *awi- "bird").

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Occam's razor (n.)

when two competing hypotheses explain the data equally well, choose the simpler. Or, as Sir William Hamilton puts it, "Neither more, nor more onerous, causes are to be assumed, than are necessary to account for the phenomena." Named for English philosopher William of Ockham or Occam (c. 1285-c. 1349), "The Invincible Doctor," who expressed it with Entia non sunt multiplicanda praeter neccssitatem.

So called after William of Occam (died about 1349): but, as a historical fact, Occam does not make much use of this principle, which belongs rather to the contemporary nominalist William Durand de St. Pourçain (died 1332). [Century Dictionary]
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occasion (n.)

late 14c., occasioun, "opportunity; grounds for action or feeling; state of affairs that makes something else possible; a happening, occurrence leading to some result," from Old French ochaison, ocasion "cause, reason, excuse, pretext; opportunity" (13c.) or directly from Latin occasionem (nominative occasio) "opportunity, appropriate time," in Late Latin "cause," from occasum, occasus, past participle of occidere "fall down, go down," from ob "down, away" (see ob-) + -cidere, combining form of cadere "to fall" (from PIE root *kad- "to fall"). The notion is of a "falling together," or juncture, of circumstances. The sense of "the time or a time at which something happens" is from 1560s.

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