To demotivate someone is to take away their wish to try. They may still come in, but they no longer feel eager or willing. Unfair bosses, low pay, or work that never gets a thank-you can demotivate a whole team. It is mostly a workplace word, a little formal; with friends people more often say that something put them off. Though actually, teachers and coaches use it too — not only offices. It is the opposite of motivate: one gives you a reason to try, the other quietly takes that reason away.