What does amain mean?
The definition of amain
A*main”, adv.
Etym: [Pref. a- + main. See 2d Main, n.]

1 With might; with full force; vigorously; violently; exceedingly. They on the hill, which were not yet come to blows, perceiving the fewness of their enemies, came down amain. Milton. That striping giant, ill-bred and scoffing, shouts amain. T. Parker.
2 At full speed; in great haste; also, at once. “They fled amain.” Holinshed.

A*main”, v. t.
Etym: [F. amener. See Amenable.] (Naut.)

To lower, as a sail, a yard, etc.

A*main”, v. i. (Naut.)
To lower the topsail, in token of surrender; to yield.