Origin

This character combines earth or ground (土, the lower part) with a sound hint (陏, the upper part) that suggests the character's Chinese pronunciation. The earth radical makes sense for the meanings because the character describes things falling down or breaking apart—physical collapse that involves earth, soil, or foundation giving way. When something degenerates or lapses, it descends or crumbles, much like earth sliding or crumbling downward. The sound component tells you roughly how to pronounce it in Chinese-influenced Japanese reading, while the earth radical points you toward the meaning of decline and downward movement.