plough
耜 centers on a farming tool used to turn soil: a plough. Its meaning is specific to this agricultural implement.
耜 combines 耒 (a plough-like farming tool) with 㠯, which likely contributes a phonetic element. The character is specifically associated with the plough.
The left side 耒 looks like a wooden plough with a handle, and the right side 㠯 resembles a blade or share. Together they form the complete plough.
For シ, imagine a plough cutting through the soil with a sharp 'shee' sound: shee -> シ, as the blade slices the earth.