This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 衠 is uncertain. It visually combines 行 (go, action) with 真 (truth, reality), but the original semantic and phonetic roles are unclear.
Since the meaning is unclear, focus on the shape: 行 wraps around 真 like a container of truth. Imagine a rare, ancient character that once meant 'pure truth in action', but treat its meaning as uncertain.
For シュン, imagine a shun (シュン) of truth: when you act (行) with true (真) intent, you shun falsehood. The sound 'shun' cues シュン.