How Did a Nod Come to Mean “Yes”?
The idea of nodding to mean “Yes” comes from the opposite of the action which, as just described, indicates a “No.”
When the young animal was anxious to accept the offered food, it made an effort to get at the food quickly. Hence, the pushing forward of the head and the open mouth (always more or less opened when you nod to indicate “Yes”) and an expression of gladness. You will notice if you see anyone nod the head to indicate “Yes” that the lips are open rather than closed, and that there is always a smile or an indication of a smile to accompany it. In other words, the nod to mean “Yes” is only another way of saying “I shall be pleased.”