To blindside someone means to surprise them with something they didn't see coming, usually something unpleasant or shocking. The word comes from the idea of being hit from your blind side, the direction you can't see.

When a teacher announces a pop quiz, she blindsides the class. When your best friend suddenly stops speaking to you without explanation, you've been blindsided. The word captures that disorienting feeling of being caught completely off guard.

In football, a blindside tackle hits the quarterback from the direction they can't see while focusing downfield. This usage shows why blindsiding feels unfair: the person being blindsided never had a chance to prepare or defend themselves. That's why blindsiding someone with bad news (like announcing you're moving away right before a big event) can damage trust, even if the news itself wasn't your fault.

You can also use blindside as a noun: “That announcement was a total blindside.”

The key element is surprise combined with impact. A surprise birthday party isn't a blindside because it's pleasant. But learning your favorite restaurant closed down when you arrive for dinner? That's a blindside.