noun
a type of warship in the 19th century protected by iron or steel armor plates
adjective
strong, secure, and unbreakable
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The contract between the two parties had ironclad clauses to ensure compliance.
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I have an ironclad alibi for where I was last night.
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His argument was so ironclad, no one could refute it.
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Her determination was like an ironclad will, nothing could deter her from reaching her goals.