This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
匰 is a rare character combining the enclosure 匚 with 單. The exact historical development is uncertain.
The outer 匚 suggests a container, and the inner 單 is complex. Picture a mysterious box whose contents are unknown, matching the uncertain meaning.
For タン, imagine a tan-colored box with the 匚 shape: tan -> タン, and the box holds something unknown.