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1.
Crazy, out of control, nuts.

In addition to being a kind of chair, a 'rocker' is a device that controls the movement of a piece of machinery. When a machine is off its rocker, it no longer works correctly.
1) You can't walk down the street in your underwear! Are you off your rocker?

2) I'm afraid Aunt Betty is really off her rocker these days.
by VAKI5 May 11, 2005
 
2.
It means acting nuts.
To be a crazy person.
Matt: Oh, I can't believe you just jumped off that 80 feet building! You must be off your rocker!
by LesleyDianna February 22, 2012