Archaic or poetic form meaning at dawn or in the early morning light.
From 'a-' prefix (at/on) plus 'dawn'; represents older English patterns where prepositions merged into adverbial forms, now preserved mainly in dialect and poetry.
Poets still use 'adawn' because it saves a syllable and sounds more archaic-mystical—it's a word that lives in fantasy novels and medieval poetry precisely because it sounds old enough to be true.
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