1) Above reproach 2) Bulletproof 3) Faultless 4) Impregnable 5) Incontestable 6) Incontestible 7) Indisputable 8) Inviolable 9) Invulnerable 10) Strong 11) Unattackable 12) Unbeatable 13) Unchallengeable 14) Untouchable 1) Immune to attack 2) Impossible to assail 3) Impregnable 4) Inviolable 5) Invulnerable 6) Secure 7) Strong 8) Unassailable logic 9) Unattackable 10) Without flaws or loopholes 
[adj] - impossible to assail
unshakable adjective without flaws or loopholes; `an ironclad contract`; `a watertight alibi`; `a bulletproof argument`
not open to attack or assault, as by military force or argument: unassailable fortifications; unassailable logic. · not subject to denial or dispute: Shakespeare's genius gives his works an unassailable position in world literature.
[SAT terms] impossible to attack