A: Michael came to our business meeting in pajamas yesterday. Everybody was shocked.

    B: I think he must have gone off his rocker.

    Note: This idiom means “crazy,” “out of one’s mind.” In this expression, rocker refers to a rocking chair. To sit in a rocking chair is a comfortable experience, but to fall off one’s rocking chair suggests that you are falling out of control.

    Hence and therefore, by extension and figuratively speaking, if we say someone is off his or her rocker, we mean to say they’re crazy, out of control, and out of their mind.