Onset, n.
Etymology: [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. Shak. Who on that day the word of onset gave. Wordsworth.
2. A setting about; a beginning. [Obs.] Shak. 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. [Obs.] Johnson.
1 other Webster entry under this spelling
Onset, v. t.
1. To assault; to set upon. [Obs.]
2. To set about; to begin. [Obs.] Carew.
Shades of meaning
Fernald has no entry of its own for onset; he lists it under attack.
He lists it without comment; the entries above explain its neighbors.