Superlative form of careful: the most careful, showing the greatest attention to detail or highest degree of caution.
From careful + -est (superlative suffix). An older or dialectal superlative form; modern English prefers 'most careful' for this longer adjective.
Old English had forms like 'carefullest' everywhere—but it gradually sounded awkward, so English speakers stopped adding -est to longer words and switched to 'most,' creating a more streamlined grammar system.
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