Definition of "insane"

What is the cause of insanity? Nobody can answer such a sweeping question as that, but we know that certain diseases, such as syphilis, break down and destroy the brain cells and result in insanity. In fact, about one-half of all mental diseases can be attributed to such physical causes as brain lesions, alcohol, toxins, and injuries. But the other half—and this is the appalling part of the story—the other half of the people who go insane apparently have nothing organically wrong with their brain cells. In post-mortem examinations, when their brain tissues are studied under the highest-powered microscopes, they are found to be apparently just as healthy as yours and mine. Why do these people go insane?

1936, Dale Carnegie, “Part 1, Chapter 2. THE BIG SECRET OF DEALING WITH PEOPLE”, in How to Win Friends and Influence People, page 41

The preposterous altruism too! […] Resist not evil. It is an insane immolation of self—as bad intrinsically as fakirs stabbing themselves or anchorites warping their spines in caves scarcely large enough for a fair-sized dog.

1918, W[illiam] B[abington] Maxwell, chapter XVI, in The Mirror and the Lamp, Indianapolis, Ind.: The Bobbs-Merrill Company

Cederstrand was all over the ice, whipping his sled around and firing pucks into the net. It's what Straschnitzki is aiming for. "He's insane. The turns, the shots. It's something I want to do some day."

2018 August 8, Bill Graveland, “'I'll get better': Paralyzed Humboldt Broncos player working to improve at sledge hockey”, in CBC‎

And fans echoed their responses. "You nailed it, girl! This movie is so special. Your work is insane! 💗," one person wrote. "Is there ANYTHING you can't do?! I'm impressed," said another.

2020 August 24, Nicol Natale, “Watch Reese Witherspoon Nail a Seriously Impressive Backflip in This Throwback Video”, in Prevention‎

On getting two quick eliminations, SypherPK cries out ecstatically: This gun is insane, it's the best gun ever! Woohoo!

2020 September 19, Saahil Agnelo Periwal, “Fortnite Chapter 2 Season 4: SypherPK claims a Victory Royale by using the 'rarest' item in the game- The Galactus Gun”, in Sportskeeda‎