Source : Webster's Revised Unabridged Dictionary (1913)

Amain \A*main"\, v. t. [F. amener. See {Amenable}.] (Naut.)
   To lower, as a sail, a yard, etc.

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

Amain \A*main"\, adv. [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.

Source : WordNet®

amain
     adv 1: at full speed; with great haste; "the children ran down the
            hill amain"
     2: with all your strength; "he pulled the ropes amain" [syn: {with
        full force}]