Off One's Nut Meaning

(chiefly Britain, dated in US, idiomatic) Insane, crazy.

Example: 1909, P. G. Wodehouse, Mike: A Public School Story, ch. 17:
  Old Smith was awfully bucked because he'd taken four wickets. I should think he'd go off his nut if he took eight ever.
1914, Zane Grey, The Rustlers of Pecos County, ch. 13:
  "[I]f I was to spring this news in Mr. Wright's hearin', why, such a sensitive, high-tempered gentleman as he would go plumb off his nut."
1918, Rex Ellingwood Beach, The Winds of Chance, ch. 11:
  "Look at them rapids ahead of us! Why, the guy that laid out this river was off his nut!"