luxuriant growth of flower
薾 describes a state of abundant, flourishing growth, specifically of flowers blooming in a lush, dense manner.
薾 combines the grass radical 艹 with 爾, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with lush plant growth.
The top grass radical 艹 covers the dense, intricate shape of 爾 below, like a thick carpet of flowers growing so luxuriantly that they form a tangled, abundant mass.
For シ, imagine a field of flowers so lush that they seem to whisper 'sheer' abundance: sheer -> シ, capturing the overwhelming growth.