I didn't know that terminology until this week. It's a business term. I must confess though, that I did understand the concept in practice and theory, but had no words for it.
Sunk costs are the idea that people won't make a new, better decision because they have spent time and energy and money on the old decision.
A common example is the person who has a large closet full of clothes they never wear. They don't get rid of them because of the investment in money, time and memories in them. Never mind that they are actually a burden and they could do something more useful with that space. (Like build that writing space they've been dreaming of.)
When we see it that clearly, we have an aha!
Why don't we dump the clothes and build the writing space we need?! Why don't we change a habit to make our lives better and therefore everyone's around us? Why are we clinging to this? Because we made it? Because we feel like we spent our lives making it we should keep trying to make it work? We obviously don't really CARE about the 'clothes.' They don't do what we need them to do anymore.
It's a brave thing to realize the new decision trumps the old and we can trim the old off in order to focus on the new, better idea. We have better information now. We are older than we were yesterday. We are wiser than we were yesterday. It's okay to re-evaluate based on new understanding.
It's okay to change our beliefs.