Appalling means shockingly bad or horrifying, something so terrible it makes you recoil in dismay. When something is appalling, it's awful enough to make you stop and think, “I can't believe this is happening.” It crosses beyond ordinary badness into something that genuinely horrifies or troubles you.

You might describe behavior as appalling when someone acts with stunning rudeness or cruelty, like a student who laughs at a classmate's mistake instead of helping. A teacher might find a student's cheating appalling, not just wrong but shocking. Living conditions can be appalling when they're dangerously unsanitary or unsafe.

The word carries a sense of moral outrage or deep concern. Appalling test scores suggest disaster, not just poor performance. An appalling mess in the cafeteria is beyond ordinary untidiness: it's shocking chaos.

Notice that appalling describes things that genuinely horrify or disgust you, not minor annoyances. Forgetting your homework is unfortunate. Copying someone else's homework and claiming it as your own is appalling. The word suggests something has crossed a line from merely bad into territory that troubles your conscience or sense of what's acceptable.