That's interesting, Lighter.

I have read a good many books on the English language (420's in the library), and some scholars talk about "drift", which are strong tendencies, perhaps unconscious, for us to talk a certain way. With the word "mischievious", we see drift which says that fancy adjectives ought to end in -ious, such as

obvious
devious
furious
curious
impecunious

The only other adjective I can think of right now which doesn't have the i is "larcenous."

I remember hearing a teacher in grade school telling us that the word is "mischievous." I believed her, but I thought it awkward.
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Re: impecunious. If I am pecunious, what am I like?