To fluster someone is to make them feel upset or agitated. There are many things that might fluster you: giving a speech in front of the whole school, solving a challenging math problem, or even getting a love note from an admirer.
If your coworker is about to give the biggest presentation of his life, you’ll probably only fluster, or upset, him if you point out that his socks don’t match. Use the adjective form, flustered, to describe someone when they’re feeling this way. Most of us get flustered once in a while, but if you’re “easily flustered,” it happens to you quite often. Don’t worry, though; that just gives you more opportunities to practice using this vocabulary word.