It translates to 'worker', & the play came out shortly before the russian revolution, so it's clear that the subtext of the term in its original use is not merely a criticism of slavery but also a marxist criticism of the exploitation of the proletariat under industrial capitalism.

(Important to note that, just like in Do Androids Dream of Electric Sheep & Blade Runner, the 'robots' in R.U.R. are not mechanical devices but instead genetically engineered synthetic humans. That helps the metaphor a great deal: rather than being a pure 'other' whose behavior might be seen as a design error, it's simply a different breed or race of humans with different heredity that makes them convenient for othering, along with some argument about hereditary traits that supposedly make them more suited to hard labor.)