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away (adv.)[away 词源字典]: late Old English aweg, earlier on weg "on from this (that) place;" see a- (1) + way (n.). Colloquial use for "without delay" (fire away, also right away) is from earlier sense of "onward in time" (16c.). Intensive use (as in away back) is American English, first attested 1818.[away etymology, away origin, 英语词源]
fire (v.): c. 1200, furen, "arouse, enflame, excite" (a figurative use); literal sense of "set fire to" is attested from late 14c., from fire (n.). The Old English verb fyrian "to supply with fire" apparently did not survive into Middle English. Related: Fired; firing.

Meaning "expose to the effects of heat or fire" (of bricks, pottery, etc.) is from 1660s. Meaning "to discharge artillery or a firearm" (originally by application of fire) is from 1520s; extended sense of "to throw (as a missile)" is from 1580s. Fire away in the figurative sense of "go ahead" is from 1775.

The sense of "sack, dismiss from employment" is recorded by 1885 (with out; 1887 alone) in American English. This probably is a play on the two meanings of discharge (v.): "to dismiss from a position," and "to fire a gun," influenced by the earlier general sense "throw (someone) out" of some place (1871). To fire out "drive out by or as if by fire" (1520s) is in Shakespeare and Chapman. Fired up "angry" is from 1824 (to fire up "become angry" is from 1798).