To overbear someone or something is to press them down until they lose — by force, by weight, or by a stronger will. It is an old, rather bookish verb. You might find it in a history text: one side overbears the other. The picture is bearing down from above, not the animal called a bear. Though actually, almost nobody uses this verb in daily talk. What people do use is overbearing, the adjective for a person who always pushes others. There is also a garden use, where a tree overbears if it grows too much fruit and tires itself — that is a different job for the same pieces, and not this sense. If you need a living word for “defeat”, you want beat, defeat, or overwhelm; keep overbear for the page it still lives on.