Fig.: to behave in a crazy. irrational way

Albert inherited a lot of money and he went off his rocker: he divorced his wife and married a girl younger than his daughter

Alberto ha ereditato un sacco di soldi e ha perso la tramontana; ha divorziato dalla moglie e ha sposato una ragazza più giovane di sua figlia

When her car went dead in the middle of the night and left her stranded on a deserted road, Cathy went off her rocker

Caterina ha perso la tramontana quando le si è fermata la macchina nel cuore della notte, in una strada deserta

It's widely accepted that this expression alludes to a rocking chair's instability, which could cause a person to fall off. Someone falls off when they lose their balance or stability, which is a metaphor for mental instability