mental illness
an action that is stupid and likely to have extremely bad results: 疯狂;失去理智
It would be insanity to expand the business now.现在扩大经营规模简直是疯了。

SMART Vocabulary: related words and phrases

Stupid and silly
absurd
absurdity
absurdly
adolescent
against your better judgmentidiom
dopey
dopily
dorky
dottiness
dotty
imprudently
in his/her/their wisdomidiom
in someone's infinite wisdomidiom
inadvisable
injudicious
subnormal
uncritical
uncritically
undiscriminating
unintelligent

insanity | American Dictionary

insanity
noun[ U ]
us/ɪnˈsæn·ɪ·t̬i/
mental illness:
The jury found him innocent on the grounds of temporary insanity.