/ ˈluːnəsi; NAmE / noun [ U ]
1.
behaviour that is stupid or crazy
SYN madness :
It's sheer lunacy driving in such weather.
2.
( old-fashioned ) mental illness
SYN madness
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WORD ORIGIN
mid 16th cent. (originally referring to intermittent insanity believed to be caused by changes of the moon): from lunatic + -acy .