So unique or valuable that nothing else could take its place if it were lost or gone.
From the prefix 'ir-' (not) plus 'replaceable' (able to be replaced), built on 'replace' from Old French 'replacer', meaning to put back in place. The prefix negates the ability to be replaced, creating a word meaning irretrievably singular.
The word gained emotional weight through the 20th century—we started using it most for sentimental items like family heirlooms or photographs, but now it applies to experiences too, reflecting how modern life makes us cherish unrepeatable moments.
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