This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 薋 is uncertain. It visually combines the grass radical 艹 with 資, but the original semantic and phonetic roles are unclear.
The top grass radical 艹 suggests a plant, and the bottom 資 (resources) might hint at a plant used for resources, but the meaning is too uncertain to build a reliable mnemonic.
For シ, imagine a rare plant (艹) that is a valuable resource (資) and is simply called 'she' by botanists: she -> シ.