Inci*den"tal*ly

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]
    

26 Moby Thesaurus words for "incidentally":
      a propos, accidentally, apropos, apropos of, as it chanced,
      by a fluke, by accident, by chance, by good fortune, by hazard,
      by the by, by the way, casually, en passant, for example,
      for some reason, fortuitously, in passing, in some way,
      par exemple, parenthetically, perchance, somehow, somehow or other,
      speaking of, unpredictably