If you do something disdainfully, you do it in a way that shows you do not respect the person or thing in front of you — you treat them as small, or as not worth your time. She waved the idea away disdainfully; he looked disdainfully at the cheap shoes. The adverb is about manner. Though actually, in real life the “disdain” is often a bit of theatre: a teenager rolling their eyes, a critic dismissing a book they barely opened. It is a sharp word. Quiet, firm, or unfriendly is not automatically disdainful; save it for when someone is clearly looking down.