stab
剚 centers on the act of stabbing or thrusting a sharp object into something.
剚 combines 事 (matter, thing) with 刂 (knife), suggesting using a knife on a matter, i.e., stabbing. The exact historical development is uncertain, but the component structure is consistent with a phono-semantic compound where 事 may have contributed the sound.
The left side 事 looks like a hand holding a tool, and the right side 刂 is a knife. Imagine using a knife to stab into a matter or object.
For シ, picture a scene where you see (シ) someone stab a target with a knife: see -> シ, and the knife 刂 does the stabbing.