Meaning : Marked by firm determination or resolution. Not shakable.
Example :
Firm convictions.
A firm mouth.
Steadfast resolve.
A man of unbendable perseverence.
Unwavering loyalty.
Synonyms : firm, steadfast, steady, stiff, unbendable, unfaltering, unwavering
Meaning : Without flaws or loopholes.
Example :
An ironclad contract.
A watertight alibi.
A bulletproof argument.
Synonyms : bulletproof, unassailable, watertight