amain
With all of one's might; mightily; forcefully, violently.
archaic, literary
His bloody vittels at his mouth, and vttred out amayne The clottred gobbets mixt with wyne, I [Odysseus] thus ſurmyſde: like lot Hangs ouer my head now, and I muſt alſo go to pot.
And in beholding how he [the cyclops Polyphemus] fed and belked vp againeAt full speed; also, in great haste.
archaic
At length the Danes beeing aſſayled on eche ſide, both a front before, and on their backes behinde, oppreſſed as it were wyth multitude, they threwe downe theyr weapons and fled amain.
Out of control.
The waggonway lay near the Windmill Hills and went down the north side of the hills to the Rivir Tine, and at the Coal steath [= staithe] Mathew Gray lived. I was about hauf way down the bank when thur was two Waggons Coming after me Amain [= broken loose and running away].
Exceedingly; overmuch.
obsolete
They whom I favour thrive in wealth amain, While Virtue, Valour, Wiſdom ſit in want.
Riches are mine, Fortune is in my hand;
Where it comes from
From a- (prefix with the sense ‘at; in; on; with’, used to show a state, condition, or manner) + main (“force, power, strength”). Main is derived from Middle English mayn (“strength”), from Old English mæġen (“strength”), from Proto-Germanic *maginą (“might, power, strength”), *maginaz (“strong”), from Proto-Indo-European *megʰ- (“to be able”).
Adapted from the Wiktionary entry amain (CC BY-SA 4.0).