Letting saliva flow from the mouth uncontrollably, or desiring something intensely.
Possibly from Old Norse 'drúla' or similar Germanic roots. The word appeared in English around the 16th century. The figurative meaning of 'wanting desperately' developed by analogy with the physical action.
Dogs drooling became such a universal symbol of desire that the image transferred to human language—'drooling over something' now means you want it so badly you've lost control, which is why Pavlov's famous dog experiment resonates across languages and cultures.
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