FOK isn't quite as vulgar as it sounds—it stands for "Fill or Kill." If you're putting in an order to buy or sell stocks or other securities, FOK is a type of order you can place. In this type of order, the broker/dealer has to fill the whole order... or kill it (not literally... they just walk away, rather than trying to fill part of the order).
Example
JoeBob places an order for a thinly traded, illiquid stock: GumboNation (GBN). GBN only trades 300,000 shares a day and the order is for half a million. The broker has to buy all 500,000 shares for JoeBob, or he's got no order to fill. He tries to fill and not get FOK'd.
FOK usually comes with a time element: FOK by noon today.