Definition of box
Box as in the expression "Send your box". This is an expression mostly used in France and mostly used live. To send your box means to put all your chips in play. The expression is synonymous with "I'm all in". It's a casual way of saying that your entire stack is in play. It is not an official expression that will be found throughout all poker communities, but it is used in live poker games in French casinos (and perhaps in other European countries where French is spoken).
Some examples
You can use the example while playing or after playing, to tell a friend about a hand for example.
He folded and I showed him my bluff.
- "On the river, I suspected my opponent was very strong. I saw him bet big and the bet was back to me. I didn't know what sizing to do, but since he looked very strong and I had the nuts, I decided to throw my box at him."