Off One's Rocker Meaning

Example :   He's off his rocker if he thinks he can tackle the whole thing alone.1960, P. G. Wodehouse, Jeeves in the Offing, chapter VIII:  â€œBertie! Your manner is strange.” “Your manner would be strange if you'd been sitting on the floor of Wilbert Cream's sleeping apartment with a chair round your neck, and Ma Cream had come in.” “Golly! Did she?” “In person.” “What did you say?” “I said I was looking for a mouse.” “Couldn't you think of anything better than that?” “No.” “And how did it all come out in the end?” “I melted away, leaving her plainly convinced that I was off my rocker.”