More awful; comparative form of awful, meaning more terrible, dreadful, or of poor quality.
From awful (Old English 'ageful' meaning full of awe) plus -er suffix for comparative. The word evolved from meaning 'inspiring awe' to 'inspiring fear or dread' to 'very bad or unpleasant.'
English speakers often use 'more awful' instead of 'awfuller' in modern speech, but the -er form follows the same pattern as 'big/bigger.' You'll hear both, though 'more awful' sounds more formal and educated.
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