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]