> Your definition doesn't work.

It is a recognized idiom in many English dialects:

> If people work very hard for long hours for very little money, you can refer to it as slave labor.

* https://www.collinsdictionary.com/dictionary/english/slave-l...

> very hard work for which people are paid very little

* https://dictionary.cambridge.org/dictionary/english/slave-la...

> (informal) work that is very hard and very badly paid

* https://www.oxfordlearnersdictionaries.com/definition/americ...

See also "working in the salt mines" where people do not literally take sodium chloride out of the ground.

* https://idioms.thefreedictionary.com/back+to+the+salt+mines