Emily Dickinson Lexicon

ON'SET, n. [on and set.]

1. A rushing or setting upon; a violent attack; assault; a storming; appropriately, the assault of an army or body of troops upon an enemy or a fort. The shout / Of battle now began and rushing sound / Of onset. – Milton.
2. An attack of any kind; as, the impetuous onset of grief. – Philips.