Third-person singular present of 'warn'; tells someone about a possible danger or problem.
From Old English 'weornian' (to be cautious, warn), related to Old Norse and Germanic roots. The word has maintained its meaning of alerting someone to danger since Anglo-Saxon times.
The word 'warn' is actually related to 'aware' and 'wary'—they all come from the same Germanic root meaning to be cautious or vigilant, so warning someone is literally trying to make them 'aware' or 'wary' of danger.
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