"Shorting-against-the-box is the act of selling short securities that you already own. For example, if you own 200 shares of FON and tell your broker to sell short 200 shares of FON, you have shorted against the box. Note that when you short against the box, you have locked in your gain or loss, since for every dollar the long position gains, the short position will lose and vice versa.

An alternative way to short against the box is to buy a put on your stock. This may or may not be less expensive than doing the short sale. The IRS considers buying a put against stock the same as shorting against the box."


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Note added at 6 mins (2006-02-06 08:51:53 GMT)
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.....sales of securities that seller does not own at the time or, if owned, are not delivered within 20 days after the sale (a short sale "against the box")....

He's just using the investment term for selling short securites that are already owned, vice those that the seller doesn't own at the time.