1 especially British English
very stupid behaviour that could be dangerous or have a very bad effect
it is madness (for somebody) to do something
It would be madness to drive all that way on your own.
Cutting down the forest is sheer madness (=completely crazy).
serious mental illness [= insanity]:
His family has a history of madness.
when someone does something without thinking clearly:
In a moment of madness, I agreed to have the party at my house.