To go crazy; to become insane. The oldest of these three expressions is “off his head,” which was current although slangy by the time Thomas Hood wrote The Turtles (1844), “He was ‘off his head.’” The word chump became British slang for “head” in the late nineteenth century; subsequently, “off his chump” was used several times by Shaw, in Pygmalion and Heartbreak House. Off one’s rocker comes from the same period, but its origin is more puzzling. One writer suggests it may indirectly allude to the elderly, associated with both rocking chairs and diminished mental capacity. Yet another variant is to go off one’s trolley, which alludes to a motorman getting off a streetcar to reposition the trolley wheel on the overhead wire that carried electric current to the car’s motor. To be disconnected from this power source came to mean becoming crazy, a usage dating from the late 1890s. With the demise of streetcars in many American cities, this expression is heard less often today.

See go off one's head.