The Collaborative International Dictionary of English v.0.48:

Indigestible \In`di*gest"i*ble\, a. [L. indigestibilis: cf. F.
   indigestible. See In- not, and Digest.]
   [1913 Webster]
   1. Not digestible; not readily soluble in the digestive
      juices; not easily convertible into products fitted for
      absorption.
      [1913 Webster]

   2. Not digestible in the mind; distressful; intolerable; as,
      an indigestible simile. --T. Warton. --
      In`di*gest"i*ble*ness, n. -- In`di*gest"i*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

indigestible
    adj 1: digested with difficulty [ant: digestible]