Unbeknownst means happening without someone's knowledge or awareness. When something occurs unbeknownst to you, it's happening but you have no idea about it.

The word often appears in stories when the audience knows something a character doesn't. Unbeknownst to the detective, the real thief had been hiding in the library the whole time. Or in real life: Unbeknownst to the birthday girl, her friends were planning a surprise party.

You can also use unbeknown, which means exactly the same thing, though unbeknownst is more common. Both words feel somewhat old-fashioned and formal, so you're more likely to encounter them in books than in everyday conversation. People rarely say “unbeknownst to me” when talking casually; they're more likely to say “without my knowing” or “I had no idea.”

It almost always needs a “to” afterward: unbeknownst to her parents, unbeknownst to the teacher, unbeknownst to anyone. It creates a sense of dramatic irony, that interesting moment when readers or listeners know something important that someone in the story doesn't.