Толковый словарь английского языка Oxford English Reference - once

adv., conj., & n. --adv. 1 on one occasion or for one time only (did not once say please; have read it once). 2 at some point or period in the past (could once play chess). 3 ever or at all (if you once forget it). 4 multiplied by one; by one degree. --conj. as soon as (once they have gone we can relax). --n. one time or occasion (just the once). Phrases and idioms all at once 1 without warning; suddenly. 2 all together. at once 1 immediately. 2 simultaneously. for once on this (or that) occasion, even if at no other. once again (or more) another time. once and for all (or once for all) (done) in a final or conclusive manner, esp. so as to end hesitation or uncertainty. once (or every once) in a while from time to time; occasionally. once or twice a few times. once-over colloq. a rapid preliminary inspection or piece of work. once upon a time at some vague time in the past. Etymology: ME anes, ones, genit. of ONE