angelica, type of iris
芷 refers to fragrant flowering plants, specifically angelica and a type of iris. The core idea is a sweet-smelling herb or bloom.
芷 combines the grass radical 艹 with 止, which likely serves as a phonetic component. The exact historical development is uncertain.
The grass crown 艹 sits above 止, like fragrant angelica or iris stopping to bloom in a field.
For シ, imagine a single iris stem standing tall: the 'she' sound of the stem swaying in the breeze cues シ.