The Collaborative International Dictionary of English v.0.48:

Inextensible \In`ex*ten"si*ble\, a.
   Not capable of being extended; not elastic; as, inextensible
   fibers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inextensible
    adj 1: not extensile [syn: nonextensile, inextensible,
           nonprotractile] [ant: extensible, extensile]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "inextensible":
   adamant, adamantine, immovable, immutable, inductile, inelastic,
   inextensile, inextensional, inflexible, intractable, intractile,
   irresilient, nonelastic, nonstretchable, stubborn, unalterable,
   unbending, unextendible, unextensible, unflexible, ungiving,
   unlimber, unmalleable, unpliable, unpliant, untractable,
   unyielding