Honeymoon 蜜月

In many parts of Europe it was traditional to supply a newly married couple with enough mead for a month, ensuring happiness and fertility. From this practice we get honeymoon or, as the French say, lune de miel. However, this etymology is not accepted by linguists.
-- From wikipedia

1546, hony moone, but probably much older, from honey in reference to the new marriage's sweetness, and moon in reference to how long it would probably last, or from the changing aspect of the moon: no sooner full than it begins to wane. Fr. has cognate lune de miel, but Ger. version is flitterwochen (pl.), from flitter "tinsel."
-- From etymonline