lunacy (n.)youdaoicibaDictYouDict[lunacy 词源字典]
1540s, "condition of being a lunatic," formed in English from lunatic + -cy. Originally in reference to intermittent periods of insanity, such as were believed to be triggered by the moon's cycle. The Old English equivalent was monaðseocnes "month-sickness."[lunacy etymology, lunacy origin, 英语词源]