selling short means: A stock that is not yours to sell. An investor may borrow stock from a broker to sell the stock if they feel the stock’s price is falling. The investor will eventually have to buy back the stock on the open markets. You might borrow 1000 shares of XYZ and then sell them for $8 per share. On Aug. 1, 1000 shares are purchased at 7 cents each. Selling short has resulted in 1000 lesser commissions and fees. (in Stock Market Dictionary)