means The trip or period of romantic happiness immediately following a wedding, before everyday reality sets in.

from A 16th-century English compound of 'honey' (the sweetness of new love) and 'moon' (a month, or a lunar cycle). The bittersweet implication, often noted by early writers, was that the affection waxed full and then waned like the moon — sweet now, but fading. The notion that it referred to a literal month of drinking honey-wine (mead) is a charming folk story with no solid evidence behind it.