Someone conniving is calculating, scheming, and shrewd — in other words, sneaky and up to no good.
Do you know anyone who is always trying to get away with things? Do they constantly look for ways to get out of trouble or work? Those kind of people are conniving. This is a word for secretive, shifty behavior. However, being conniving isn't the worst thing in the world — it's negative, but you probably wouldn't say a murderer is conniving. It's usually reserved for con men, shady business moguls, and manipulative social climbers.