It's a strong word that emphasizes the extreme nature of the insanity.
Here are some examples of how it can be used:
* "Er ist im Wahnsinn!" (He is insane!)
* "Das ist Wahnsinn!" (That's crazy!)
* "Sie ist wahnsinnig geworden." (She has gone mad.)
Keep in mind that the intensity of the word "Wahnsinn" can vary depending on the context.