Superlative form of cheerful; the most full of cheer and happiness.
From 'cheerful' plus the superlative suffix '-est.' Like 'cheerfuller,' this form is archaic; modern English prefers 'most cheerful' for multi-syllable adjectives.
The way English abandoned '-est' endings for longer adjectives is still happening—we're gradually shifting toward 'most X' instead of 'X-est,' which linguists call 'periphrasis,' and it makes our language easier to parse.
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