To warn; an archaic or poetic form meaning to caution or alert someone to danger.
From ā- prefix + warn (Old English 'warnian,' from Germanic 'war-' meaning 'aware' or 'cautious'). The prefix emphasizes the action of warning.
The ā- prefix in Middle English (as in 'awarn,' 'ablaze,' 'afoot') was originally an intensifier—it let speakers punch up the meaning, and we still do this with prefixes today.
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