Not showing thanks for something good that someone gave you or did for you. The key word is SHOWING. "Ungrateful" is almost always a judgement made from the outside: someone helped you, and you did not say thank you, or you complained, or you acted as if it was owed to you. That last part is where the real force of the word lies — an ungrateful person treats a gift as their right. Because of this, the word carries strong moral disapproval, much stronger than most un- words. Parents say it to children ("after everything we've done for you"), and it is one of the sharper things you can call someone in English. It is also used of behaviour rather than people: an ungrateful reply, an ungrateful silence. Two useful extras. English has the fixed insult "you ungrateful little…" which is genuinely angry, not playful. And the noun form is "ingratitude" — not "ungratefulness," which exists but sounds clumsy — one of those places where English switches prefix between the adjective and the noun.