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

AMAIN is a 5 letter word that starts with A.

1. \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.
   

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

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