To appall means to shock or horrify someone so deeply that they feel dismayed and disturbed. When something appalls you, it goes beyond just surprising you: it troubles you in a way that's hard to shake off.

If you discovered that your best friend had been spreading cruel rumors about another classmate, you might be appalled by their behavior. The condition of an animal shelter with sick, neglected animals would appall most visitors. Learning that a trusted adult had lied repeatedly might leave you feeling appalled and disappointed.

The word suggests a kind of shock mixed with moral disgust or distress. You might be startled by a loud noise, but you're appalled by something that violates your sense of what's right or acceptable. When doctors in the 1800s first saw the appalling conditions in crowded city hospitals, their shock motivated them to push for better sanitation and care.

Something that appalls you doesn't just catch you off guard: it makes you recoil because it seems so wrong, unfair, or terrible. The feeling lingers because what you've witnessed conflicts with your values or expectations about how people should behave or how things should be.