Overweening is a negative term meaning arrogant or excessive. People can be described as having overweening pride or overweening ambition. It's too much and not good.
Confidence and pride are okay in moderation. Overweening means having too much of it though so that it overtakes the rest of your personality, and not in a good way. If your football team has won every game of the season, they need to watch that they don't become overweening and start playing games as if they have already won.