(FINANCE) the situation in which a derivatives trader with a short position is
wrong about the behavior of the market. Having sold shares of
stock he doesn't own, he is now compelled to buy them at a higher
price than he sold them for (in order to reimburse whomever he borrowed the shares from).
If the short position was taken by writing naked options (i.e., issuing call options of stock the trader doesn't happen to have), then the trader has to buy shares of underlying stock in order to
honor the options.
It's extremely expensive for traders to have to cover their shorts.