If work is backbreaking, it is so hard on the body that you feel worn out, as if your back might break. Think of lifting heavy boxes all day, digging, farming in the sun, or carrying water up stairs. It is a strong, slightly informal word: you use it when you want someone to feel how painful the work is, not when you are filling in a form. Though actually people stretch it. Office workers sometimes call a long week backbreaking even when nobody's back was in danger. The older, central idea is physical work that leaves you aching.