barefooted
跣 centers on being barefoot: walking or standing without shoes or sandals.
跣 combines the foot radical 足 with 先, which likely contributes the sound and may suggest the idea of going ahead or being exposed. The exact historical development is uncertain.
The foot radical 足 beside 先 (ahead) suggests a foot going forward without protection: barefoot.
For セン, imagine a barefoot person stepping on a sen coin (銭, せん) and feeling the cold metal: sen -> セン.