Narrow openings or cracks in something; or the sounds made by coins or glasses touching each other.
From Middle English 'chinke' (a narrow opening). Possibly related to 'chine' (a ravine or crevice). The sound meaning developed because that's what happens when you fill small gaps—they echo.
The 'chink in the armor' phrase comes from medieval warfare—literally, a tiny gap in a knight's armor could be fatal, making 'chink' a perfect word for small vulnerabilities that can have huge consequences.
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