fool, make a fool of
蚩 centers on foolishness and mockery: a fool, or the act of making a fool of someone.
蚩 combines 虫 (insect) with a top component that may be related to 之 or 屮. The exact historical development is uncertain, but the character is associated with foolishness or contempt.
Picture a worm (虫) under a silly hat (㞢) acting like a fool. The worm is making a fool of itself.
For シ, imagine a fool pointing at the worm and saying 'She!' (シ) in mockery, making a fool of the worm.