In Saki's Beasts and Super-Beasts he refers to "Nuts" and "Super-Nuts". For example, we have
“You are not going to be what they call a Nut, are you?” she inquired with some anxiety, partly with the idea that a Nut would be an extravagance which her sister’s small household would scarcely be justified in incurring, partly, perhaps, with the instinctive apprehension that a Nut, even in its embryo stage, would refuse to carry parcels.
in The Dreamer and
He was a youngish man of ordinary appearance, quiet of dress and unobtrusive of manner, and he could never wholly rid himself of the idea that a fierce light of public scrutiny beat on him as though he had been a notability or a super-nut.
in A Holiday Task.
I suspect this usage would have briefly fashionable, prior to the First World War, when the stories were first published in the Morning Post.
So what is meant by "nut"?