This idiom is generally used to refer to work which is physically taxing to the point of feeling as though the core of one's body, the back, will break. It is often an exaggeration for effect, but in situations where somebody is required to lift and move heavy loads, it can take on a more literal meaning. Though it is usually used in the above context, 'back-breaking work' can also figuratively refer to any type of arduous activity. If a job or task requires a lot of running around, or is simply mentally draining or demoralizing, it will still have a pronounced physical effect, even if it is just to feel extremely tired. For that reason, this expression can be used in a professional context that does not necessarily involve manual labour. For example, you could say: "It took months of back-breaking work, but we finally got the business up and running." Phrases that share a similar meaning are 'work your fingers to the bone,' and 'do the donkey work.'