This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
甆 combines 兹 (top) and 瓦 (bottom). The exact historical development is uncertain, but it likely relates to pottery or earthenware given the 瓦 component.
The top 兹 looks like a tangled mass, and the bottom 瓦 is a tile or earthenware. Imagine a potter shaping a tangled lump of clay into a ceramic vessel.
For シ, picture a potter shaping a ceramic tile and saying 'she' as she works: she -> シ, and the tile 瓦 takes form under her hands.