crowd
侁 centers on a group of people moving together: a crowd or throng. The idea is of many individuals in motion as one mass.
侁 combines the person radical 亻 with 先 (advance, precede). The exact historical development is uncertain, but it likely expresses people moving forward in a group.
A person 亻 leading the way 先, with many others following close behind: a crowd surging forward.
For シン, imagine a shin-high crowd of people pressing forward: shin -> シン, and the crowd moves as one.