Something that cannot keep going — not forever, and often not even for long — because it uses up what it needs, or costs more than it brings in, or wears people out. It works in three big areas. In the environment: "unsustainable fishing" takes fish faster than they can breed, so one day there are none left. In money: "unsustainable debt" or "unsustainable spending" means the numbers only work for a while and then collapse. In work and life: "an unsustainable workload", "an unsustainable pace" — you can do it this month, but you will break. The picture underneath all three is the same, and it is the useful thing to hold on to: something is being taken out faster than it is being put back. Note that unsustainable does not mean "bad right now". A company growing 200% a year, a runner sprinting, a student sleeping four hours a night — all may be doing fine today. The word is a prediction about the future, and it is a common way to criticise something without saying it is wrong: "the plan is not wrong, it is simply unsustainable". It is formal and belongs to news, business and environmental writing rather than daily chat. In Taiwan you will meet its positive twin everywhere — 永續 — on company reports and government posters.