In a way that shows careful attention and concentration on something.
From the adjective 'attent' (now archaic), derived from Latin 'attendere' meaning 'to stretch toward' or 'to attend.' The suffix '-ly' converts the adjective to an adverb form indicating manner.
This word is basically extinct in modern English, replaced by 'attentively,' but it shows how English constantly swaps out old words for new ones—'attently' felt too similar to other words and lost out in the competition between synonyms.
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