Webster's Revised Unabridged Dictionary · 1913

Onset

Onset , noun

[On + set.]

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

Onset , transitive verb

1.
To assault; to set upon. [Obsolete]
2.
To set about; to begin. [Obsolete] — Carew