Mischievous is the dictionary-approved form of the adjective meaning causing mischief. But the misspelled mischievious, with that extra i toward the end, is so common that some dictionaries have listed it as a variant. And it’s not new. The OED lists instances of mischievious going back to the 17th century. So while mischievous is the standard spelling and is safer in formal writing, using mischievious is not a serious usage problem.
Examples
Mischievious is rare in 21st-century published writing because spell-check catches it, but it does appear from time to time—for example:
We were good friends, and I will always remember him for the warm, sincere and yet mischievious grin. [Morning Sentinel]
She remembered his mischievious side . . . [The Star Ledger]
But mischievous is far more common in editorially scrupulous publications—for example:
It is Dionysus, the mischievous god of revelry. [Financial Times]
My guess is Ms. Shemy will find some mischievous ways to make it work. [New York Times]
Tongue-in-bearded-cheek, Ben Hudson has a typically mischievous take on where his retirement will leave the Western Bulldogs. [Sydney Morning Herald]
