To move your head up and down to show agreement or understanding, or to drop your head forward when falling asleep.
From Middle English 'nodden,' possibly from Old English 'hnodan' or Germanic roots. The origin is uncertain but may be related to sudden movement. It's been used for head gestures since medieval times.
Every culture uses head nodding to mean 'yes,' but in Bulgaria and some parts of Greece, nodding means 'no' and shaking your head means 'yes'—so the same gesture means opposite things!
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