plough, spade
耡 centers on a farming tool for turning soil: a plough or spade. Both meanings describe the same kind of heavy digging implement.
耡 combines 耒 (a plough-like farming tool) with 助, which likely contributes the sound and may reinforce the idea of assistance in tilling.
The left side 耒 is a farming tool, and the right side 助 means help. Together they suggest a tool that helps turn the soil: a plough or spade.
For ジョ, imagine a farmer named Joe who uses a heavy plough: Joe -> ジョ, and Joe's spade helps him till the field.