This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 匬 is uncertain. It combines the enclosure 匚 with the inner component 俞, but the original semantic and phonetic roles are unclear.
Since the meaning is unclear, focus on the shape: a container 匚 holding something inside (俞). Imagine a mysterious box whose contents are unknown.
For ユ, imagine you opening the mysterious box: you -> ユ, and you peer inside the 匚 enclosure.