Etymology
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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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causality (n.)

c. 1600, "that which constitutes a cause," from causal + -ity. From 1640s as "the relation of cause to effect."

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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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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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inefficacy (n.)
"want of force or virtue to produce the desired effect," 1610s, from Late Latin inefficacia, from inefficacem (nominative inefficax), from in- "not, opposite of" (see in- (1)) + Latin efficax "powerful, effectual, efficient" (see efficacy).
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efficacy (n.)
1520s, from Latin efficacia "efficacy, efficiency," from efficax (genitive efficacis) "powerful, effectual, efficient," from stem of efficere "work out, accomplish" (see effect (n.)). Earlier in same sense was efficace (c. 1200), from Old French eficace (14c.), from Latin efficacia; also efficacite (early 15c.), from Latin efficacitatem.
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