>To be mentally unstable / crazy
>Behave in a strange or silly manner
This idiom doesn’t discriminate. It doesn’t have to be ‘your’ rocker. It can be his, her, or theirs.
The origin of this idiom is most likely a mechanical one. Within many machines, a rocker is a sort of regulator which helps control its performance. If this rocker slips or becomes misaligned, the machine will work incorrectly, if at all.
Example
I’m going to go off my rocker if I have to hear that song one more time!
I think his boss is a little off his rocker these days.
You must be off your rocker if you think that’s a good idea!