What is the definition and origin of "Mom Sang Dad Sang"?

It means' mother', but generally it doesn't refer to a specific person, but to that group.

There is a Japanese cultural background here. Under normal circumstances, many women will quit their jobs and go home after they get married and have children. When they have children for a period of time, they will have extra free time and often organize volleyball teams like ママさん. . Generally speaking, "ママさん" refers to the above-mentioned people who are organized for a certain purpose (plural).

For reference only.