Off one’s rocker/off one’s trolley
These two expressions are coupled because they both mean the same thing and both derived at the same time from the same source, namely America. They both mean crazy or off one’s head and derive c. 1897, from the once familiar sight of trolley men re-connecting the trolley arms to overhead electric wires. These arms were connected to the electric wires by means of a ‘rocker’ or curved wheel. When not connected, the trolley car was not able to function properly and both expressions were swiftly applied to people who displayed the same characteristics. There is another school of thought that believes off one’s rocker is derived from simply falling off a rocking chair but the trolley explanation is more favoured by etymologists.