If someone is dutiful, they do what is expected of them and behave in a loyal, obedient way. A dutiful son visits; a dutiful employee stays to the end of the meeting. It sounds a little formal, and sometimes a little cool. Though actually, the word does not always mean the person is glad. It can mean they are doing the right thing because they should, not because their heart is running ahead of them — a dutiful smile, a dutiful letter of thanks. That shade is easy to miss if you only learn “good and loyal.” It is about duty as obligation (what you owe), not duty as a customs tax. Common, but more at home in stories, family talk, and slightly careful English than in slang.