smelling like a sheep
羶 centers on the strong, distinctive odor of sheep or mutton. The meaning is specific to that animal smell, not a general term for fragrance or stench.
羶 combines 羊 (sheep) with 亶, which likely contributes the sound and may relate to the idea of spreading or extending, possibly referring to the way the smell of sheep spreads. The exact historical development is uncertain.
The left side is a sheep 羊, and the right side 亶 looks like a container with something spreading out. Imagine a sheep pen where the strong smell of sheep spreads from the container-like enclosure.
For セン, picture a hundred (百, ひゃく) sheep in a pen, and their smell is so strong you can sense it from far away: sense -> セン.