leftovers, residue, remains
剩 centers on what is left over after the main part is taken or used: leftovers, residue, or remains. It describes the surplus or remainder that persists.
剩 combines 乘 (to ride, multiply) with 刂 (knife), suggesting the idea of cutting or dividing and leaving a surplus. The exact historical development is uncertain, but the component relationship points to a leftover or remainder.
The left side 乘 can suggest multiplying or piling up, and the right side 刂 is a knife. Imagine cutting a large pile and seeing the leftovers scattered around: 剩 is what remains.
For ジョウ, picture a chef named Joe cutting a pile of food and leaving the leftovers: Joe -> ジョウ, and the remains are 剩.