Fill or Kill. FOK is a way to place an order to trade securities. Either the broker/dealer has to fill the entire order - or kill it. JoeBob places an order for a thinly traded, illiquid stock - GumboNation, ticker: GBN. GBN only trades 300,000 shares a day and the order is for half a million - the broker has to deliver all 500,000 shares to JoeBob - or he gets no order to fill. He tries to fill and not get FOK'd. FOK usually comes with a time element: FOK by noon today.