The Collaborative International Dictionary of English v.0.48:

Resistless \Re*sist"less\, a.
   1. Having no power to resist; making no opposition. [Obs. or
      R.] --Spenser.
      [1913 Webster]

   2. Incapable of being resisted; irresistible.
      [1913 Webster]

            Masters' commands come with a power resistless
            To such as owe them absolute subjection. --Milton.
      [1913 Webster] -- Re*sist"less*ly, adv. --
      Re*sist"less*ness, n.
      [1913 Webster]