If you say that someone is certifiably insane, you are emphasizing that you think they are behaving in a very silly or stupid way.

Some of the people on these websites sound certifiably insane.

He would have to be certifiably mad to do anything that might jeopardize his position.

Are you absolutely, certifiably nuts? 你绝对是疯了对吗？

This is one of the most certifiably insane decisions the government has ever taken.

These people weren't only irresponsible, they were certifiably crazy.

See also: certifiably (OFFICIALLY)