To close a window (typically by clicking a button marked with an X symbol).
To cross out with a series of the letter X, or with scribbles or lines.
To crash; to fall over (especially in board sports such as surfing, skateboarding, etc.).
To physically erase (writing, computer data, etc.).
To do away with; to cause to disappear.
To destroy (especially, a large number of people or things); to obliterate.
To knock (a surfer) off their board.
To fall off one's surfboard.
To exhaust; to tire out.