kind of weed, faithfulness
藎 is a rare kanji with two seemingly unrelated meanings: a kind of weed and faithfulness. The connection is unclear, so treat it as a character with a concrete plant sense and an abstract moral sense.
藎 combines the grass radical 艹 with 盡, which likely provides the sound. The exact historical development is uncertain, and the link between the weed and faithfulness meanings is not clearly established.
The grass top 艹 suggests a plant, and the bottom 盡 looks like a container being emptied. Picture a weed that is so faithful it keeps growing back even after you try to pull it all out.
For シン, imagine a faithful weed that grows on a shin (shin bone) shaped mound: shin -> シン, and the weed stays true to its spot.