If you are appalled, something has shocked you in a bad way. It is not a fun surprise. It is a strong feeling of “that is terrible,” mixed with upset, and often a bit of disgust. You might be appalled by cruel treatment, by a cruel joke, by filthy conditions, or by news that feels deeply unfair. The word is used in speech and in writing; it is not slang, and it is not stiff either. The feeling sits in the person: I am appalled by this, or I was appalled to hear that. Though actually, some speakers stretch it. They say they are appalled at a late train or a messy room, which is a bit dramatic, but you will hear it. Older, more careful use keeps it for things that really wound the sense of what is right or decent. Younger informal talk sometimes uses it as a loud “I cannot believe this.”