So this is a funny wee idiom.
How do you use it?
Imagine you’re watching your friend do something totally crazy, you can comment on the madness by saying to your fellow onlooker (the person who is watching with you): she’s off her rocker!
It tends to be a stand alone phrase, meaning that you don’t really use it within a long sentence. It’s more of a short expression you’d use to comment on someone’s action.
Let’s have another example:
You and your friend are at the mall and she sees a guy she has a crush on (that she likes). She decides to make a grand gesture and take control of the announcer’s microphone to declare her love for the guy. You can tell her, « Oh my goodness, you’re off your rocker. »
This idiom is generally used for funny situations rather than serious ones; you shouldn’t use it to speak of someone who has real mental health problems.
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