1.  Anything in the shape of a half moon. [1913 Webster]

2.  A figure in the form of a crescent, bounded by two intersecting arcs of circles. [1913 Webster]

3.  A fit of lunacy or madness; a period of frenzy; a crazy or unreasonable freak. [1913 Webster]
"These dangerous, unsafe lunes i' the king." [1913 Webster]

lune, n. Geom. a crescent-shaped figure formed on a sphere or plane by two arcs intersecting at two points.

Etymology:
F f. L luna moon