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. 
Audio (US) (file)
- 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.