E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To make a knight by giving him a blow. Dr. Tusler says, The ancient method of knighting was by a box on the ear, implying that it would be the last he would receive, as he would henceforth be free to maintain his own honour. The present ceremony is to tap the shoulder with a sword. (Anglo-Saxon, dubban, to strike with a blow.)