The Collaborative International Dictionary of English v.0.48:

Doubtable \Doubt"a*ble\, a. [OF. doutable, L. dubitabilis, from
   dubitare. Cf. Dubitable.]
   1. Capable of being doubted; questionable.
      [1913 Webster]

   2. Worthy of being feared; redoubtable. [Obs.]
      [1913 Webster]