rank odour of sheep or goats
羴 centers on the strong, unpleasant smell characteristic of sheep or goats. It is a specific sensory term for that rank animal odor.
羴 is composed of three 羊 (sheep) components, suggesting an intensified sheep smell. The exact historical development is uncertain, but the tripling of the sheep radical likely reinforces the idea of a strong odor.
Three sheep stacked together: imagine a pen crowded with sheep, and the air thick with their rank smell. The triple sheep visually amplifies the odor.
For セン, picture a strong scent of sheep wafting through the air: scent -> セン, and the triple sheep makes the smell unforgettable.