Crazy; mentally unsound; extremely foolish. So,if you say that someone is off their rocker, you mean that that person is behaving in a very strange or silly way or having or showing a very abnormal or sick state of mind.
Bedlam / Bonker etc. are another word which is having similar meaning.
Also, someone who is off (one’s) rocker is also referred to as mad as a hatter also.
Example
I’m going to go off my rocker if I have to hear that song one more time!
I think Jane’s grandmother is a little off her rocker these days.
You must be off your rocker if you think that’s a good idea!
Not sure if the daredevil is courageous or off his rocker for trying such a stunt.