The Collaborative International Dictionary of English v.0.48:

Comer \Com"er\, n.
   One who comes, or who has come; one who has arrived, and is
   present.
   [1913 Webster]

   All comers, all who come, or offer, to take part in a
      matter, especially in a contest or controversy. "To prove
      it against all comers." --Bp. Stillingfleet.
      [1913 Webster]