The superlative form of 'grateful' (most grateful), formed by adding '-est' instead of using 'most grateful.'
From 'grateful' plus the superlative suffix '-est.' Like 'gratefuller,' this is an older or non-standard form competing with modern 'most grateful.'
Shakespeare and his contemporaries would have used 'gratefullest' without hesitation, but modern English has decided 'most grateful' sounds better—it's a perfect example of how languages slowly reshape their grammar rules, and some old forms stick around longer in poetry or formal writing.
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