Etymology
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causation (n.)

1640s, "act of causing or producing," noun of action from cause (v.), or else from Medieval Latin causationem (nominative causatio) "action of causing" (in classical Latin "excuse, pretext"), from Latin causa "a cause, reason" (see cause (n.)). Meaning "relation of cause to effect" is from 1739.

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efficient (adj.)
late 14c., "making, producing immediate effect, active, effective," from Old French efficient and directly from Latin efficientem (nominative efficiens) "effective, efficient, producing, active," present participle of efficere "work out, accomplish," from assimilated form of ex "out" (see ex-) + facere "to do" (from PIE root *dhe- "to set, put"). Meaning "productive, skilled" is from 1787. Related: Efficiently.
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causative (adj.)

early 15c., "effective as a cause or agent," from Old French causatif, from Latin causativus, from causa "a cause, reason" (see cause (n.)). Meaning "expressing causation" is from c. 1600; hence the noun, in grammar, "a form of a noun or verb expressing causation" (1824).

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workmanlike (adj.)
"efficient, no-nonsense," 1739, from workman + like (adj.).
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efficiency (n.)

1590s, "power to accomplish something," from Latin efficientia "efficient power; efficiency; influence" (from efficientem; see efficient) + -cy. In mechanics, "ratio of useful work done to energy expended," from 1858. Attested by 1951 as short for efficiency apartment (itself from 1917).

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inefficient (adj.)
1748, "not producing or incapable of producing the desired effect," from in- (1) "not, opposite of" + efficient. Related: Inefficiently.
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rustler (n.)

1820, "one who or that which rustles" (a leaf, a bird), agent noun from rustle (v.). The American English meaning "cattle thief" is by 1882; earlier it meant "active, efficient person" (1872).

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coefficient (n.)

also co-efficient, c. 1600, "that which unites in action with something else to produce a given effect," from co- + efficient. Probably influenced by Modern Latin coefficiens, which was used in mathematics in 16c., introduced by French mathematician François Viète (1540-1603). As an adjective, "acting in union to the same end," from 1660s. Related: Coefficiency.

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etiology (n.)
also aetiology, aitiology, "science of causes or causation," 1550s, from Late Latin aetiologia, from Greek aitiologia "statement of cause," from aitia "cause, responsibility" (from PIE *ai-t-ya-, from root *ai- (1) "to give, allot;" see diet (n.1)) + -logia "a speaking" (see -logy). Related: Etiologic; etiological.
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efficacious (adj.)
"sure to have the desired effect" (often of medicines), 1520s, from Latin efficaci-, stem of efficax "powerful, effectual, efficient," from stem of efficere "work out, accomplish" (see effect (n.)) + -ous. Related: Efficaciously; efficaciousness.
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