Nonstandard or archaic past tense of 'grind'; modern English uses 'ground' instead.
From Old English 'grindan.' While regular English verbs add '-ed' for past tense, 'grind' is an irregular verb that uses 'ground' as its past tense. 'Grinded' represents either historical usage, dialect, or nonstandard speech.
English speakers constantly try to make irregular verbs regular—the fact that people still say 'grinded' shows how the language is always trying to simplify itself, but 'ground' stubbornly refuses to disappear.
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