1398, "making," from Old French, from Latin efficientem (nominative efficiens), preposition of efficere "work out, accomplish" (see effect). Meaning "productive, skilled" is from 1787. Efficiency apartment is first recorded 1930, American English. 
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- Making good, thorough, or careful use of resources; not consuming extra. Especially, making good use of time or energy.
2013 June 1, “A better waterworks”, The Economist, volume 407, number 8838, page 5 (Technology Quarterly):
- An artificial kidney these days still means a refrigerator-sized dialysis machine. Such devices mimic […] real kidneys […] . But they are nothing like as efficient, and can cause bleeding, clotting and infection—not to mention inconvenience for patients, who typically need to be hooked up to one three times a week for hours at a time.
- An efficient process would automate all the routine work.
- Our cleaners are almost too efficient: they throw away anything left out on a desk.
- Using a particular proportion of available energy.
- The motor is only 20% efficient at that temperature.
- Causing effects; producing results.
- The efficient cause is the working cause.
- third-person plural future active indicative of efficiō