As freely applied to babies and small children as it is to animals, we all recognize the meaning of the word when we see it written down. It means to be naughty, or to misbehave, but in a harmful or playful manner. When we tell people or pets off for being mischievous, though, we usually mangle the pronunciation of the word.
As frequently misspelled as it is mispronounced, something about the word structure makes you feel like there's another 'i' or 'e' in there somewhere, and the word comes out as "miss-chee-vee-us", stretching it out across for syllables when only three are called for. There's no high vowel after that 'v', and the "chie" sound comes out exactly as it does in "chief", so when you put it all together you get "miss-chev-us". Confusing, isn't it? What a mischievous little word.