Dejectedly tells you how someone does a thing: in an unhappy, disappointed, low way. He walked dejectedly. She smiled dejectedly — a smile with no light in it. The feeling is the same as dejected; this form just hangs on the verb. Use dejected for a person or a face (He was dejected / a dejected look). Use dejectedly when there is already an action to describe. It is a little formal, the sort of word a story uses when it wants you to see the body as well as the mood.