Something taxing takes a lot out of you — a lot of effort, energy, patience or attention. A taxing journey, a taxing exam, a taxing day at work, a taxing conversation with a difficult relative. Notice the flavour: it does not mean simply difficult in the way a hard sum is difficult. It means draining. The thing pulls energy out of you the way a tax pulls money out of your pocket — and that really is where the meaning comes from, the picture of a demand being made on your reserves until they run low. So it works best for things that go on for a while: the marathon, the week of care, the long meeting. It is a slightly formal, rather British-sounding word, common in writing and in careful speech. You will also hear it used with an odd politeness, in the negative, to reassure someone: 'It's not a taxing job' — meaning don't worry, it won't wear you out. It is used both before a noun (a taxing schedule) and after a linking verb (the trip was taxing).