truth, reality, Buddhist sect
眞 centers on what is genuine and unadorned: truth, reality, or the authentic essence of something. The Buddhist sect sense is a specific application of that idea of ultimate truth.
眞 is the traditional form of 真. The character is thought to combine 匕 (a spoon or person) with 目 (eye) and a lower element, possibly suggesting something seen clearly or directly, but the exact historical development is uncertain.
The top 匕 looks like a tool, and 目 is an eye. Imagine using a tool to examine something closely with your eye, revealing its true nature. 眞 is the truth you see when you look carefully.
For シン, picture a shin (shin bone) being examined for truth: shin -> シン, and the eye 目 sees the real bone structure.