Synonyms of the word 
DEFENSIBLE → DEFENDABLE - INVULNERABLE
defensible
- adj. (of an installation etc) capable of being defended against armed attack.
- adj. (of an argument etc) capable of being justified.
defendable
- adj. Capable of being defended.
invulnerable
- adj. Incapable of being injured; not vulnerable.
- adj. Unanswerable; irrefutable.
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