The act of short selling securities that you already own. This results in a neutral position where your gains in a stock are equal to the losses. For example, if you own 100 shares of ABC and you tell your broker to sell short 100 shares of ABC, you have shorted against the box. An alternative to short selling against the box is to buy a put on your stock. This may or may not be less expensive than doing the short sale.
Also known as "shorting against the box".
Taobiz explains Short Sell Against the Box
Before 1997, the sole rationale for shorting against the box was to delay a taxable event. According to tax laws that preceded 1997, owning both long and short positions in a stock meant that any papers gains from the long position would be removed temporarily due to the offsetting short position. All in all, the net effect of both positions is zero, meaning that no taxes need to be paid.
Let's say that you have a big gain on some shares of ABC. You think that ABC has reached its peak and you want to sell. However, the tax on the capital gain may leave you under-withheld for the year and subject to penalties. Perhaps the next year you expect to make a lot less money, putting you in a lower bracket and causing you to want to take the gain at that time. However, the Taxpayer Relief Act of 1997 no longer allows short selling against the box as a valid tax deferral practice. Under TRA97, capital gains or losses incurred from short selling against the box are not deferred. The tax implication is that any related capital gains taxes will be owed in the current year.