Water or liquid that has been used to wash buck or deer hides, or possibly a term related to processing such hides.
From 'buck' (deer) and 'wash.' This appears to be a technical term from hide-tanning operations, though it's quite obscure in modern usage.
In hide-processing operations, the water used to wash hides—buckwash—was not wasted; it was actually used as a fertilizer or in other industrial processes, making nothing in pre-industrial manufacturing truly disposable.
Complete word intelligence in one call. Free tier — 50 lookups/day.