Someone who is overexcited is very excited and not behaving in a sensible way — talking too fast, not listening, maybe knocking things over. Parents, teachers, and people talking about pets use this word constantly. It is usually about children, dogs, and crowds before a concert, not about a calm adult sharing good news. The feeling itself is not bad; the over- says there is too much of it for the moment. Though actually, when adults use it about other adults it can sound a little talking-down, as if the person should calm down. You will also hear get overexcited and an overexcited crowd.