The Collaborative International Dictionary of English v.0.48:

Indefectible \In`de*fect"i*ble\, a. [Pref. in- not + defectible:
   cf. F. ind['e]fectible.]
   Not defectible; unfailing; not liable to defect, failure, or
   decay.
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         An indefectible treasure in the heavens. --Barrow.
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         A state of indefectible virtue and happiness. --S.
                                                  Clarke.
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