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.