E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The month after marriage, or so much of it as is spent away from home; so called from the practice of the ancient Teutons of drinking honey-wine (hydromel) for thirty days after marriage. Attila, the Hun, indulged so freely in hydromel at his wedding-feast that he died.
It was the custom of the higher order of the Teutons to drink mead or metheglin (a beverage made from honey) for thirty days after every wedding. From this comes the expression to spend the honeymoon.W. Pulleyn: Etymological Compendium, § 9, p. 142.