The Collaborative International Dictionary of English v.0.48:

Infrangible \In*fran"gi*ble\, a. [Pref. in- not + grangible: cf.
   F. infrangible.]
   1. Not capable of being broken or separated into parts; as,
      infrangible atoms.
      [1913 Webster]

            [He] link'd their fetlocks with a golden band
            Infrangible.                          --Pope.
      [1913 Webster]

   2. Not to be infringed or violated.
      [1913 Webster]

WordNet (r) 3.0 (2006):

infrangible
    adj 1: difficult or impossible to break or separate into parts;
           "an infrangible series"
    2: not capable of being violated or infringed; "infrangible
       human rights" [syn: absolute, infrangible, inviolable]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "infrangible":
   chip-proof, coherent, cohesive, firm, firm as Gibraltar,
   fractureproof, impartible, indiscerptible, indissoluble,
   indissolvable, indivisible, infusible, inseparable, insoluble,
   made of iron, nonbreakable, rigid, shatterproof, solid, sound,
   stable, staunch, stout, sturdy, unbreakable, undividable, unified,
   unshatterable