The most romantic word in English started as a warning. When honeymoon first appears, it is a reminder that the sweetness of a new marriage will not last.
The earliest record is from 1546, in John Heywood's collection of English proverbs. One speaker answers talk of a happy new marriage with "it was yet but hony moone." In other words, give it time.
The first explanation came in 1552, in Richard Huloet's English-Latin dictionary. He called it a term "proverbially applied to such as be new married," who love each other exceedingly at the start. The Oxford English Dictionary's early sense compares that love to the moon, which is no sooner full than it begins to wane. Liberman is not sure Huloet should be trusted on this. Thomas Blount's 1656 glossary put it in one line: "it is hony now, but it will change as the Moon."
So the honey is the sweetness and the moon is the clock. Anatoly Liberman, writing for Oxford University Press, notes that moon once also meant "month." A variant, honey-month, was in use by 1640. Samuel Johnson's 1755 dictionary defined honeymoon as the first month after marriage, "when there is nothing but tenderness and pleasure." Liberman adds a caution: Huloet may simply have repeated someone's joke, and nobody knows who coined the word.
A popular story says the word comes from an old northern European custom of drinking mead, a honey wine, for a month after the wedding. That theory was put forward in the 19th century and has since been challenged. Liberman rejects it. Whatever people drank elsewhere, such a custom hardly explains a word coined in 16th-century England.
The wider sense came quickly. By 1579 John Stubbs was using honeymoon for any early period of goodwill, the sense now applied to new governments and new bosses.
The trip came much later. Wedding travel seems to have started in early 19th-century Britain as the "bridal tour," when well-off couples visited relatives who had missed the ceremony. From the 1820s the French called it a voyage in the English fashion. Even French lune de miel, used by Voltaire in 1747, is probably a translation of the English word, which is two centuries older.