1. To cause one to topple over as a result of a very forceful impact. The cyclist wasn't going very fast at all, but he still knocked me sideways when he ran into me. The child was standing at the edge of the beach when a large wave knocked her sideways.

2. To shock, surprise, astonish, or bewilder one, especially in a distressing or upsetting manner. I know the death of his mother really knocked Tom sideways. It knocked everyone in the company sideways when Olivia announced that she was leaving.