The Collaborative International Dictionary of English v.0.48:

Fossilize \Fos"sil*ize\, v. t. [imp. & p. p. Fossilized; p.
   pr. & vb. n. Fossilizing.] [Cf. F. fossiliser.]
   1. To convert into a fossil; to petrify; as, to fossilize
      bones or wood.
      [1913 Webster]

   2. To cause to become antiquated, rigid, or fixed, as by
      fossilization; to mummify; to deaden.
      [1913 Webster]

            Ten layers of birthdays on a woman's head
            Are apt to fossilize her girlish mirth. --Mrs.
                                                  Browning.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fossilize \Fos"sil*ize\, v. i.
   1. To become fossil.
      [1913 Webster]

   2. To become antiquated, rigid, or fixed, beyond the
      influence of change or progress.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fossilize
    v 1: convert to a fossil; "The little animals fossilized and are
         now embedded in the limestone" [syn: fossilize,
         fossilise]
    2: become mentally inflexible [syn: fossilize, fossilise]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "fossilize":
   age, anneal, antiquate, become extinct, become obsolete, calcify,
   callous, case harden, cornify, date, fade, firm, fust, grow old,
   harden, indurate, lapidify, lithify, lose currency, molder,
   obsolesce, ossify, outdate, perish, petrify, rust, steel,
   superannuate, temper, toughen, vitrify