The novelist Kingsley Amis - Lucky Jim (1954) uses the word though he puts it in quotation marks. It appears three times in the novel.
"Well, if you drink as much as that you must expect to feel a bit off-colour the next day, mustn't you?" She drew herself upright in her seat in a schoolmarm attitude. He remembered his father, who until the war had always worn stiff white collars, being reproved by the objurgatory jeweller as excessively "dignant" in demeanour. This etymological sport expressed for Dixon exactly what he objected to in Christine. He said rather coldly: "Yes, I must mustn't I?"
How well really the Callaghan girl had behaved, in spite of her stand-offishness at times, and how sound her suggestion had been. That, and her laughing fit, proved that she wasn't as "dignant" as she looked. He remembered uneasily the awful glow of her skin, the distressing clarity of her eyes, the immoderate whiteness of those slightly irregular teeth.
"Not that I think there's anything foolish in coming to see you. Oh, I just don't seem to be able to put it in any way that sounds at all sensible." Little by little and intermittently, she was adopting her "dignant" tone and physical attitude.
There is no OED entry for dignant