Overjoyed means extremely happy — not a small smile, but the feeling when news is so good it fills you. Parents are overjoyed at a birth; you might be overjoyed to hear a friend is coming home. It is a warm, slightly careful word: at home in a card, a speech, or a kind conversation, less at home in slang. You use it as an adjective: she was overjoyed; they were overjoyed to see us. People almost never say I overjoy as a verb in ordinary talk. Over- here means “completely, more than the usual amount,” and joy is the happiness. It is stronger than glad or pleased, and a little more formal than so happy.