This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
覟 combines 志 (will, intention) and 見 (see). The exact historical development is uncertain, but it likely relates to seeing with intention or observing carefully.
The left side 志 suggests intention or will, and the right side 見 means to see. Together, 覟 could be imagined as looking at something with focused attention.
For シ, imagine seeing (見) a sheet (シート) of paper with your intentions (志) written on it: sheet -> シ.