affair, matter, work
叓 centers on a task or piece of business: an affair, a matter to handle, or work to be done.
The historical origin of 叓 is uncertain. It visually contains 又 at the bottom, but the exact development of the upper part is unclear.
The bottom 又 looks like a hand, and the upper part resembles a simplified tool or document. Imagine a hand holding a work order: an affair or matter to take care of.
For シ, picture a busy office worker saying 'She' handles every affair: She -> シ, and she is always on top of her work.