What's the difference between i.e. and e.g.?
early 15c., "nut-like," from nut + -y (2). Meaning "crazy" is from 1898 (see nuts); earlier "amorous, in love (with)," 1821. Related: Nuttiness.
Crazy; very eccentric; nuts: I was just about nutty, I was so lonely (1898+)