Past tense of grit: to clench your teeth together firmly, or to spread grit (small stones) on a surface.
From Old English grytt, related to grinding particles. Originally meant the coarse particles themselves, but evolved to mean clenching teeth (like grinding) or spreading those particles.
When you say 'gritted his teeth,' you're describing an action that literally sounds like what it means—the hard /t/ sounds mimic that crunching! This is called onomatopoeia in disguise.
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