1. To be particularly shy, diffident, or timid by nature. Primarily heard in US. My brother is a very sweet, warm-hearted man who wouldn't say boo to anyone. How can you suspect him of committing this crime?

2. To not say a word (to anyone); to keep quiet. Primarily heard in US. The neighbor's daughter is just the cutest little thing, but she wouldn't say boo the whole time we were there. I trust you didn't say boo to those coppers about the little side business we've been running here?