奥

The word "oku" primarily describes a point or area that is farthest from the observer or the entrance, emphasizing depth and distance along a specific axis. Unlike "ushiro" (behind/back), which refers to the area physically behind an object or person, "oku" refers to the inner depths of a space, such as the back of a room, a deep forest, or the interior of a drawer.

When used metaphorically, it refers to the hidden, innermost layers of a person's heart, mind, or the essence of a subject. It often carries a nuance of mystery, sincerity, or profoundness, as seen in the word "okubukai" (profound/deep).

In a social context, "oku" (usually as "okusan") is a polite term for someone else's wife. This usage originates from the traditional Japanese architectural layout where the wife's primary domain was the inner part (the "oku") of the house, away from the public entrance. It is important to note that this term is used only for others' wives; one would never use it to refer to their own wife in a formal setting.