[pauke]

No, in Czech the meaning really is more specific and forced labor is not an overly bad translation, it's work done by serfs in medieval times. It can be used to describe any work as heavy, but that's either in joke, or when used by people geographically close to Poland (typically referring to mining).

[bee_rider]

Is there some historical reason for this?

I vaguely remember that in English, we have words like “cattle,” French etymology, and “cow” Germanic, and the speculation is that it is because the aristocracy were French for much of England’s history (so, the French word is used to refer to cows as a sort of abstract resource to be considered in bulk).

[pauke]

I believe it's a similar thing. Semi-educated guess based on historical facts: Because of various (not least religious) reasons, Czech-speaking intelligentsia pretty much ceased to exist mid-17th century (fact, replaced by German/Latin) and only actual serfs spoke Czech, work and robota became defacto synonym (speculation). And when it became fashionable and cool to speak Czech 100-200 years later for the city-dwellers (fact), they probably felt the need to differentiate whatever they were doing as a job from the definitely uncool farmers of the countryside (speculation).