The Collaborative International Dictionary of English v.0.48:

Inseparable \In*sep"a*ra*ble\, a. [L. inseparabilis: cf. F.
   ins['e]parable. See In-, and Separable.]
   [1913 Webster]
   1. Not separable; incapable of being separated or disjoined.
      [1913 Webster]

            The history of every language is inseparable from
            that of the people by whom it is spoken. --Mure.
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            Liberty and union, now and forever, one and
            inseparable.                          --D. Webster.
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   2. (Gram.) Invariably attached to some word, stem, or root;
      as, the inseparable particle un-.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inseparable
    adj 1: not capable of being separated; "inseparable pieces of
           rock"