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1540s, hony moone, but probably much older, "indefinite period of tenderness and pleasure experienced by a newly wed couple," from honey (n.) in reference to the new marriage's sweetness, and moon (n.) 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. French has cognate lune de miel, but German version is flitterwochen (plural), from flitter "tinsel" + wochen "week." In figurative use from 1570s. Specific sense of "post-wedding holiday" attested from c.1800; as a verb in this sense from 1821. Related: Honeymooned; honeymooning.
An early stage in a project or activity before there are problems and disagreements: honeymoon is over when the new employee has his bearings