Something unstoppable cannot be stopped, no matter what anyone does to stop it. The '-able' here is doing something worth noticing: it is not about the thing's own power alone, but about whether other people can put a stop to it. So the picture behind the word is always someone trying and failing. You use it in three main ways. For people: an unstoppable player, singer, or salesperson keeps winning and nobody can block them. For things that move or grow: an unstoppable train, an unstoppable rise in prices, an unstoppable spread of a disease. And for feelings that come out of you whether you want them or not: an unstoppable urge to laugh, unstoppable tears. In real speech it is usually a compliment and usually a big, warm exaggeration — 'she's unstoppable this season' does not mean literally no one can ever beat her; it means she is on a run and looks that way right now. Sports commentators, business writers, and coaches love it. It is common in ordinary conversation too, so unlike many long English words this one is safe to say out loud. When it describes something bad — an unstoppable virus, an unstoppable decline — the tone turns from admiring to frightened, but the meaning stays the same.