Yup, your explanation makes the most sense F.Steve.

Épormyable is definitely not a word, but, as I understand it, that what was M. Gauvreau was all about. It just didn't take. I'd have to read the play to know what he meant the word to mean.

If I have to hazard a guess, I'd say "Ép" he would have taken from épeurant (scary) or épouvantable (abominable or scary) and "ormy" from formidable (mighty, formidable or impressive).