When you're miffed, you're offended or annoyed. It would be hard not to feel miffed if your entire family forgot your birthday.
The informal miffed is great for describing a feeling of irritation or impatience. Your teacher might be miffed if you keep showing up late to class, and you'll feel equally miffed if he doesn't believe your excuse that the bus is always late. Miffed came along in the early nineteenth century, from miff, "fit of ill humor," which is probably imitative of the disgusted sound you might make when you're fed up or offended.