The comparative form of 'grateful' (more grateful), formed by adding '-er' instead of using 'more grateful.'
From 'grateful' (from Old English 'grace' + '-ful') plus the comparative suffix '-er.' This is an older or dialectal form that competes with the more modern 'more grateful.'
English is actually fighting with itself over how to make comparatives—'gratefuller' sounds old-fashioned now, but it follows the same pattern as 'slower' or 'smaller,' showing how we've gradually shifted toward 'more grateful' without fully abandoning the '-er' form.
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