To add something extra free of charge.
If you take the large size, we'll throw in a set of tea towels.
To include in a calculation.
To quit, to fold.
To inject, as a fluid.
A set-piece where the ball is thrown back into play
A throw of the ball back into play by the boundary umpire. The umpire faces away from the players and throws it in over his or her shoulder (so as to clearly not favour either side by where exactly it's thrown).
The start time of a Gaelic football match, comparable to kick off in other games.
An extra item added free of charge; a product that is thrown in with another.