If everyone at the lunch meeting is vying for the last roast beef sandwich, but you grab it first, you preempt your colleagues from getting it. Let them eat liverwurst! Preempt means to displace or take something before others can.
Preempt combines the Latin prefix prae-, "before," with emere, "to buy." Think old-fashioned land grabs and midnight madness sales. It can also mean to replace one thing with another that’s more important. For instance, if the president is speaking or the football game is running long, brace yourself: the network just might preempt your favorite show.