Source : Webster's Revised Unabridged Dictionary (1913)

Undestroyable \Un`de*stroy"a*ble\, a.
   Indestructible.

Source : WordNet®

undestroyable
     adj 1: very long lasting; "less durable rocks were gradually worn
            away to form valleys"; "the perdurable granite of the
            ancient Appalachian spine of the continent" [syn: {durable},
             {indestructible}, {perdurable}]
     2: not capable of being destroyed