chump

To be crazy. In the UK, "chump" is a slang term for "head." You're off your chump if you think that plan will work.

A tiny, trifling, and/or inconsequential amount of money. To most people, $2,000 is a lot to spend on anything, but to the country's mega rich, it is merely chump change. I'm only getting paid chump change for all this hard work I'm doing.

Crazy or insane. He must be off his chump to pay that much money for that old car. Am I completely off my chump for starting a business in this economy?

A trivial sum of money, a trivial matter. For example, Dave was sick of working for chump change; he wanted a decent salary, or Don't put that on the agenda; it's chump change. This expression uses chump in the sense of "a fool or sucker who should be ignored." [Slang; 1960s] Also see chicken feed.

Also, off one's nut or rocker or trolley or chump . Crazy, out of one's mind, as in You're off your head if you think I'll pay your debts, or I think Jerry's gone off his nut over that car, or When she said we had to sleep in the barn we thought she was off her rocker, or The old man's been off his trolley for at least a year. The expression using head is colloquial and dates from the mid-1800s, nut has been slang for "head" since the mid-1800s; rocker, dating from the late 1800s, may allude to an elderly person falling from a rocking chair; trolley, also dating from the late 1800s, may be explained by George Ade's use of it in Artie (1896): "Any one that's got his head full of the girl proposition's liable to go off his trolley at the first curve." The last, chump, is also slang for "head" and was first recorded in 1859.

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.