off one's rocker

Prepositional phrase

1. (slang) Crazy; insane.
   • He's off his rocker if he thinks he can tackle the whole thing alone.
   • “Bertie! 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...1960, P[elham] G[renville] Wodehouse, chapter VIII, in Jeeves in the Offing, London: Herbert Jenkins, →OCLC, page 86