It is the unrecoverable outlay that has already occurred when you put effort into something, including money, energy, time, and so on. Looking toward the future, it is silent—a past that needs no consideration: the meal you've already eaten a few bites of, the feelings you've invested halfway, the position you've strived at for years.
Sunk cost triggers a psychological effect: the more you invest, the more obsessed you become with the cost, the harder it is to stop, and the harder it is to turn back. The dish you ordered tastes bad, but you've eaten half of it—leaving it unfinished would be a waste. Your partner isn't a good fit, but you've been together for so many years, so you keep making do. When you encounter difficulty and hesitate, waver, and pace back and forth, as long as a voice reminds you of the four words 'sunk cost,' you will naturally conclude that you should stop investing. If you continue, you're ignoring advice, stubbornly clinging to error, refusing to shed tears until you see the coffin.
Nowadays, the term 'sunk cost' is always used after something that appears, to onlookers, to be a mistake, in order to persuade someone to stop. Over time this affects people's judgment of objective facts—and the facts might be: if the food isn't good, you put down your chopsticks, but then you go hungry at work in the afternoon. You have a quarrel in daily life and decide the person isn't right for you, and as a result you miss out on a relationship. The overall situation looks bad, so you think the company has no future, and shortly after you leave, the situation improves and the company goes public. We shouldn't assume, just because sunk cost exists, that the action itself must be wrong and something to stop, right? You have to look at the matter itself.
Your understanding of the objective situation and your assessment of your own capabilities. For example, if a ship develops a fault and is about to sink, and you are an experienced captain, then if you discover that objectively it is no longer worth trying to save, or that subjectively you know you lack the ability to save it, the ship has already become a sunk cost in your eyes. But if you discover that objectively it can still be salvaged, or that subjectively you have the ability to save it, then even if others think it has become a sunk cost, you should continue investing without hesitation and bring it back. So whether something is a sunk cost also depends on whether your own judgment is sound; what is a sunk cost in someone else’s eyes may not be one in yours. Uneaten food does not necessarily have to be thrown away—you can pack it up and take it home to feed to kittens and puppies. If the bus has not arrived for half an hour, should you continue waiting? You can open the app and check its location first. Experiences that have already passed can also help you become more mature and sensible at some point in the future, preparing you to encounter them again. Every decision should place greater emphasis on your own assessment of your abilities and the likely outcome, choosing what benefits you rather than losing your ability to make your own judgment just because someone mentions sunk costs.