Etymology
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thwart (adv.)
c. 1200, from a Scandinavian source, probably Old Norse þvert "across," originally neuter of thverr (adj.) "transverse, across," cognate with Old English þweorh "transverse, perverse, angry, cross," from Proto-Germanic *thwerh- "twisted, oblique" (source also of Middle Dutch dwers, Dutch dwars "cross-grained, contrary," Old High German twerh, German quer, Gothic þwairhs "angry"), altered (by influence of *thwer- "to turn") from *therkh-, from PIE root *terkw- "to twist." From mid-13c. as an adjective.
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thwart (v.)
"oppose, hinder," mid-13c., from thwart (adv.). Related: Thwarted; thwarting.
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athwart (adv.)
"crosswise, from side to side," late 15c., from a- (1) + thwart (v.). In nautical use, "across the line of a ship's course."
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*terkw- 
Proto-Indo-European root meaning "to twist."

It forms all or part of: contort; distort; extort; extortion; nasturtium; queer; retort; thwart; torch; torment; torque (n.) "rotating force;" torsion; tort; torticollis; tortuous; torture; truss.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit tarkuh "spindle;" Latin torquere "to twist;" Old Church Slavonic traku "band, girdle;" Old High German drahsil "turner," German drechseln "to turn on a lathe;" Old Norse þvert "across," Old English þweorh "transverse, perverse, angry, cross," Gothic þwairhs "angry."
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cross-cut (adj.)

"used for cutting crosswise," 1820, from cross (adv.) + cut (v.). As a verbal phrase, "to cut transversely," from 1590s. An old name for a cross-cut saw was thwart-saw (mid-15c.).

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checkmate (v.)

late 14c., figurative, "to thwart, frustrate;" see checkmate (n.). As a verb in chess, from 1789. Related: Checkmated; checkmating.

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stymie (v.)

1857, in golf, from stymie (n.) "condition in which an opponent's ball blocks the hole" (1834); of uncertain origin, perhaps from Scottish stymie "person who sees poorly," from stime "the least bit" (early 14c.), itself of uncertain origin. General sense of "block, hinder, thwart" is from 1902. Related: Stymied.

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encumber (v.)
early 14c., "burden, vex, inconvenience," from Old French encombrer "to block up, hinder, thwart," from Late Latin incombrare, from in- "in" (from PIE root *en "in") + combrus "barricade, obstacle," probably from Latin cumulus "heap" (see cumulus). Meaning "hinder, hamper" is attested in English from late 14c. Related: Encumbered; encumbering.
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cross (v.)

c. 1200, "make the sign of a cross as an act of devotion," from cross (n.) and in part from French croiser. Sense of "to go across, pass from side to side of, pass over" is from c. 1400; that of "to cancel by drawing a line over or crossed lines over" is from mid-15c.

From late 14c. as "lie across; intersect;" also "place (two things) crosswise of each other; lay one thing across another." From early 15c. as "mark a cross on." Meaning "thwart, obstruct, hinder, oppose" is from 1550s. Meaning "to draw or run a line athwart or across" is from 1703. Also in Middle English in now-archaic sense "crucify" (mid-14c.), hence, figuratively, crossed "carrying a cross of affliction or penance."

Sense of "cause to interbreed" is from 1754. In telegraphy, electricity, etc., in reference to accidental contact of two wires on different circuits or different parts of a circuit that allows part of the current to flow from one to the other, from 1884. Meaning "to cheat" is by 1823.

Cross my heart as a vow is from 1898. To cross over as euphemistic for "to die" is from 1930. To cross (someone's) path "thwart, obstruct, oppose" is from 1818. Of ideas, etc., to cross (someone's) mind "enter into" (of an idea, etc.) is from 1768; the notion is of something entering the mind as if passing athwart it.

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oar (n.)

"long wooden lever for propelling a boat," Middle English or, from Old English ar, from Proto-Germanic *airo (source also of Old Norse ar, Danish aare, Swedish åra), a word of unknown origin. Apparently unrelated to the IE root that is the source of Latin remus "oar," Greek eretēs "rower," eretmos "oar," English row (v.) and rudder. As "oar-like appendage of an animal," 1580s.

A long oar, used occasionally to assist a vessel in a calm, is a sweep, and is operated by two or more men. Small oars are sculls; one rower wielding a pair, sitting midlength of the thwart. ["Knight's American Mechanical Dictionary," 1884]
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