In a happy, optimistic, or pleasant manner. Done with good spirits and enthusiasm.
From Middle English 'chere' meaning face or expression, derived from Old French 'chiere'. The suffix '-ful' was added to create 'cheerful' meaning full of good cheer, then '-ly' to form the adverb.
The word 'cheer' originally referred to one's facial expression or countenance, which is why we still say someone has 'good cheer' - it literally meant having a pleasant face. Interestingly, the connection between facial expression and inner emotional state was so strong that the word evolved to mean the emotion itself.
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