A mystery to first-time visitors to India, it eventually grows on most foreigners who find it endearing and quaint.
It is also called ‘the head wobble’ – a common form of nonverbal communication that has been perfected in India over centuries. Apparently there is a historical reason for it: It evolved from fearfulness by the general population when spoken to by their kings or other authoritarian figures to respond with ‘no’. So instead of saying no, they tilted their head side-to-side which was noncommittal but counted as an answer to be interpreted by the other.
This video makes an earnest attempt to explain the different types of head shakes and their meaning.
Credit to Max and Bhadrena