unshakable

(s) marked by firm determination or resolution; not shakable
- firm convictions
- a firm mouth
- steadfast resolve
- a man of unbendable perseverence
- unwavering loyalty
firm, steadfast, unbendable, unfaltering, unwavering, steady, stiff,
(s) without flaws or loopholes
- an ironclad contract
- a watertight alibi
- a bulletproof argument
bulletproof, watertight, unassailable,

firm steadfast unbendable unfaltering unwavering steady stiff bulletproof watertight unassailable

unshakably