Etymology
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travail (n.)
"labor, toil," mid-13c., from Old French travail "work, labor, toil, suffering or painful effort, trouble; arduous journey" (12c.), from travailler "to toil, labor," originally "to trouble, torture, torment," from Vulgar Latin *tripaliare "to torture," from *tripalium (in Late Latin trepalium) "instrument of torture," probably from Latin tripalis "having three stakes" (from tria "three;" see three + palus "stake" (from PIE *pakslo-, suffixed form of root *pag- "to fasten"), which sounds ominous, but the exact notion is obscure. The verb is recorded from late 13c. in English, from the verb in Old French.
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travois (n.)
type of American Indian transport, 1847, said to be ultimately from a Canadian Indian pronunciation of travail.
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travel (v.)
late 14c., "to journey," from travailen (1300) "to make a journey," originally "to toil, labor" (see travail). The semantic development may have been via the notion of "go on a difficult journey," but it also may reflect the difficulty of any journey in the Middle Ages. Replaced Old English faran. Related: Traveled; traveling. Traveled (adj.) "having made journeys, experienced in travel" is from early 15c. Traveling salesman is attested from 1885.
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labor (n.)

c. 1300, "a task, a project" (such as the labors of Hercules); later "exertion of the body; trouble, difficulty, hardship" (late 14c.), from Old French labor "toil, work, exertion, task; tribulation, suffering" (12c., Modern French labeur), from Latin labor "toil, exertion; hardship, pain, fatigue; a work, a product of labor," a word of uncertain origin. Some sources venture that it could be related to labere "to totter" on the notion of "tottering under a burden," but de Vaan finds this unconvincing. The native word is work.

Meaning "body of laborers considered as a class" (usually contrasted to capitalists) is from 1839; for the British political sense see labour. Sense of "physical exertions of childbirth" is attested from 1590s, short for labour of birthe (early 15c.); the sense also is found in Old French, and compare French en travail "in (childbirth) suffering" (see travail). Labor Day was first marked 1882 in New York City. The prison labor camp is attested from 1900. Labor-saving (adj.) is from 1776. Labor of love is by 1797.

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*pag- 
also *pak-, Proto-Indo-European root meaning "to fasten."

It forms all or part of: Areopagus; appease; appeasement; compact (adj.) "concentrated;" compact (n.1) "agreement;" fang; impact; impale; impinge; newfangled; pace (prep.) "with the leave of;" pacific; pacify; pact; pagan; page (n.1) "sheet of paper;" pageant; pale (n.) "limit, boundary, restriction;" palette; palisade; patio; pawl; pax; pay; peace; peasant; pectin; peel (n.2) "shovel-shaped instrument;" pole (n.1) "stake;" propagate; propagation; travail; travel.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pasa- "cord, rope," pajra- "solid, firm;" Avestan pas- "to fetter;" Greek pegnynai "to fix, make firm, fast or solid," pagos "pinnacle, cliff, rocky hill;" Latin pangere "to fix, to fasten," pagina "column," pagus "district;" Slavonic paž "wooden partition;" Old English fegan "to join," fon "to catch seize."
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sore (adj.)
Old English sar "painful, grievous, aching, sad, wounding," influenced in meaning by Old Norse sarr "sore, wounded," from Proto-Germanic *saira- "suffering, sick, ill" (source also of Old Frisian sar "painful," Middle Dutch seer, Dutch zeer "sore, ache," Old High German ser "painful," Gothic sair "pain, sorrow, travail"), from PIE root *sai- (1) "suffering" (source also of Old Irish saeth "pain, sickness").

Adverbial use (as in sore afraid) is from Old English sare but has mostly died out (replaced by sorely), but remains the main meaning of German cognate sehr "very." Slang meaning "angry, irritated" is first recorded 1738.
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labor (v.)
late 14c., "perform manual or physical work; work hard; keep busy; take pains, strive, endeavor" (also "copulate"), from Old French laborer "to work, toil; struggle, have difficulty; be busy; plow land," from Latin laborare "to work, endeavor, take pains, exert oneself; produce by toil; suffer, be afflicted; be in distress or difficulty," from labor "toil, work, exertion" (see labor (n.)).

The verb in modern French, Spanish, and Portuguese means "to plow;" the wider sense being taken by the equivalent of English travail. Sense of "endure pain, suffer" is early 15c., especially in phrase labor of child (mid-15c.). Meaning "be burdened" (with trouble, affliction, etc., usually with under) is from late 15c. The transitive senses have tended to go with belabor. Related: Labored; laboring.
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