Completely certain, unbreakable, or protected against attack or criticism. Originally referred to warships protected with iron armor plates.
Compound of 'iron' + 'clad' (clothed), first used in the 1860s to describe armored warships during the American Civil War. The metaphorical sense of 'unbreakable' or 'certain' developed from the ships' reputation for being nearly indestructible.
The ironclad warships of the Civil War revolutionized naval warfare and gave us a perfect metaphor for absolute certainty. When we say something is 'ironclad,' we invoke the image of armor so strong that no weapon can penetrate it - the ultimate expression of unbreakable protection.
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