Wiktionary
a. which may not be attacked
WordNet
adj. able to withstand attack; "an impregnable fortress"; "fortifications that made the frontier inviolable" [syn: impregnable, inviolable, secure, strong, unassailable]
Usage examples of "unattackable".
Even if Gregoire were within his rights in bringing an action against Gervais, it would be idiotic for him to do so, because far above any petty private interest, there is the interest of all of us, the interest of the family, which is to remain, united, compact, and unattackable, if it desires to continue invincible.
The resisting power of numbers, the unattackable stolidity of a great multitude, was the haunting fear of his sinister loneliness.
Now this is an intolerable thing, although the virtuous and unattackable paterfamilias encourages it.