The Collaborative International Dictionary of English v.0.48:

Incidental \In`ci*den"tal\, a.
   Happening, as an occasional event, without regularity; coming
   without design; casual; accidental; hence, not of prime
   concern; subordinate; collateral; as, an incidental
   conversation; an incidental occurrence; incidental expenses.
   [1913 Webster]

         By some, religious duties . . . appear to be regarded .
         . . as an incidental business.           --Rogers.

   Syn: Accidental; casual; fortuitous; contingent; chance;
        collateral. See Accidental. -- In`ci*den"tal*ly,
        adv. -- In`ci*den"tal*ness, n.
        [1913 Webster]

              I treat either or incidentally of colors. --Boyle.
        [1913 Webster]