If you’re the one making prank phone calls at 3 am and you can’t resist short-sheeting your camp counselor’s bunk, you're mischievous. If your dog likes to decorate your house with streamers of toilet paper, he's mischievous, too.
You can be mischievous in harmless, playful ways, or you might also use the word mischievous to describe behavior that's more than just a little naughty. If you post online some embarrassing photos of an acquaintance, your more generous friends might describe your behavior as mischievous, although others might accuse you of being downright mean. Keep in mind that the word has only three syllables: MIS-chuh-vuhs.