1. without flaws or loopholes

    e.g. an ironclad contract
           a watertight alibi
           a bulletproof argument

    Synonym: unassailablewatertightbulletproof

2. marked by firm determination or resolution
    not shakable

    e.g. firm convictions
           a firm mouth
           steadfast resolve
           a man of unbendable perseverence
           unwavering loyalty

    Synonym: firmsteadfaststeadystiffunbendableunfalteringunwavering