The word has been in use in English since the 16th century. 'Honey' conveys a sense of sweetness and 'moon' suggests a period of one month. Put together, they mean that period immediately after a marriage during which the couple's relationship is at its sweetest and most tender. There is also a suggestion that honey is an aphrodisiac, so newly-weds might have tended to eat it in order to maximise their early marital activities! A more cynical suggestion is that it implies that - just as the moon is no sooner full than it starts to wane - so will the affections of the pair. Take your pick!