Lewis Carroll's account of portmanteau words

Take the two words "fuming" and "furious." Make up your mind that you will say both words, but leave it undecided which you will say first. Now open your mouth and speak. If your thoughts incline ever so little towards "fuming," you will say "fuming-furious;" if they turn, by even a hair's breadth, toward "furious," you will say "furious-fuming," but if you have that rarest of gifts, a perfectly balanced mind, you will say "frumious."

From preface to The Hunting of the Snark