Superlative form of cheeful; the most cheerful or the fullest of cheer.
Cheeful + superlative suffix '-est'. Traditional English superlative formation, though modern English typically uses 'most cheerful' instead.
Old English loved its '-est' superlatives (like 'fastest' and 'brightest'), and while we've moved toward 'most' for longer adjectives, single-syllable adjectives still cling to '-est'—cheefullest is a linguistic fossil!
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