This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
腨 combines ⺼ (flesh) with 耑, which likely contributes the sound. The exact historical development is uncertain.
The left side ⺼ suggests a body part, and the right side 耑 looks like a sprout above a mountain. Picture a fleshy part of the leg, perhaps the calf, as a rounded mound of flesh.
For セン, imagine a centipede crawling up the fleshy calf: cent -> セン, and the many legs tickle the muscle.