with sharp attention and enthusiasm; very actively interested in something.
From 'keen' (sharp, eager), which comes from Old English 'cene' meaning bold or fierce. The adverbial '-ly' suffix was added to create the adverb form.
A 'keen observer' catches details others miss—this word describes that almost predatory alertness, which is why it's often used for detectives, scientists, and anyone watching for hidden patterns.
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