01 n. A rushing or setting upon; an attack; an assault; a storming; especially, the assault of an army.
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1.
A rushing or setting upon; an attack; an assault; a storming; especially, the assault of an army.“The onset and retire Of both your armies.” — Shak.“Who on that day the word of onset gave.” — Wordsworth.
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2.
A setting about; a beginning; -- used especially of diseases or pathological symptoms.“There is surely no greater wisdom than well to time the beginnings and onsets of things.” — Bacon.
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3.
Anything set on, or added, as an ornament or as a useful appendage.[Obs.]