back-breaking 
adjective
(Of manual labour) physically demanding: a day’s back-breaking work...
- In poverty-stricken neighbourhoods of the city, the vast majority of children must work, often doing back-breaking manual labour for a few pounds a day.
- Monday meant a return to the fields and back-breaking labour.
- Those capable of working were not sent to the gas chambers, but put to back-breaking labour under horrific conditions.
Synonyms
gruelling, arduous, strenuous, onerous, punishing, murderous, crushing, Herculean, demanding, exacting, taxing, formidable, exhausting, draining, laborious, burdensome, tough, stiff, uphill, heavy
informal killing
British informal knackering
archaic toilsome