In a manner that shows you are paying close attention and ready to respond quickly to something.
From 'alert' (from Italian 'all'erta' meaning 'on the watchtower') plus the adverb suffix '-ly', forming an adverb meaning 'in an alert manner.'
The word 'alert' actually comes from Italian soldiers stationed on high ground watching for enemies—that's why being alert means being watchful and on guard!
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