To appal someone is to make them feel very shocked and upset — not a small surprise, but a moral or emotional jolt, often with a taste of disgust. “I was appalled” is the shape you will hear most; the plain present (this appals me) is a little less common. It is fairly strong and fairly standard: news, school essays, and serious talk all use it. This is the British spelling; American English usually writes appall, with two l’s. Though actually, people sometimes weaken it in casual talk (“I was appalled by the queue”) when they only mean annoyed — careful speakers still keep it for things that really should shock you.