Working extremely hard or laboring intensively at something difficult or exhausting.
From 'slave,' ultimately derived from the ethnonym 'Slav,' referring to Slavic peoples who were enslaved during medieval times. The sense of 'hard work' comes from the brutal nature of slavery.
It's disturbing that 'slaving away' became a metaphor for hard work—it shows how the horror of slavery became so embedded in language that we use it casually today, though most people don't realize the dark history behind it.
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