A non-standard or archaic double comparative form meaning 'more better,' though this usage is generally considered grammatically incorrect in modern English.
From 'better' (already comparative of 'good') plus the comparative suffix '-er,' creating a redundant form. This was more acceptable in Middle and Early Modern English but is now considered a mistake.
Betterer is a linguistic 'crime' that shows how our brains naturally want to follow patterns—even though 'better' is already a comparative, English learners instinctively want to add '-er' because that's how comparatives normally work, revealing the rules our brains use to generate language.
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