off one's chump

“Ay, sure ’ave ur; and wot the devil I be to do agin that there Snooks, as ’ll lie through a brick wall, I beant able to say. I be pooty nigh off my chump wot wi’ one thing and another.”

1883, Richard Harris, The Humourous^([sic]) Story of Farmer Bumpkin′s Lawsuit, Gutenberg eBook #30551

1890, Catherine Martin, An Australian Girl, 2002, Margaret Ellen Allen (biographical information), Rosemary Campbell (introduction and notes; editor), University of Queensland Press, page 90, It put him off his chump entirely. He went completely to the bad.

Such luck as he was going for was literally impossible, and he was regarded as “off his chump,” as someone put it expressively.

1891, The Australian journal: A Weekly Record of Literature, science, and Art, volume 26, page 477

What with the shock of him going in this awful rash way, and thinking myself a made man by that chance, I was nearly off my chump for a week.

1899 September – 1900 July, Joseph Conrad, chapter VI, in Lord Jim: A Tale, Edinburgh; London: William Blackwood and Sons, published 1900, →OCLC

I'm going away. He's off his chump, he is. I dont want no balmies teaching me.

1912 (date written), [George] Bernard Shaw, “Pygmalion”, in Androcles and the Lion, Overruled, Pygmalion, London: Constable and Company, published 1916, →OCLC, Act II, page 125