Surreptitious means done secretly or sneakily, especially because you know others wouldn't approve. When you take a surreptitious glance at your friend's paper during a test, you're trying to look without anyone noticing. When a student makes a surreptitious trip to the cookie jar before dinner, they're hoping no one will catch them.

The word captures that particular kind of secretiveness where you're actively trying to avoid detection. It involves being deliberately sneaky and hiding your actions. A spy might make a surreptitious exit from a building, slipping out while guards aren't watching.

Surreptitious behavior usually feels wrong because you're hiding what you're doing. If you wouldn't need to be sneaky about it, it probably wouldn't be surreptitious. That's why the word often appears in stories about spies, thieves, or people breaking rules: surreptitious actions are the ones you really don't want anyone to see.