It's pretty close to a terrible response which is just the reason it won't work, such as "That'll take too long". Often it's a valid problem but one that can be

It's pretty close to a terrible response which is just the reason it won't work, such as "That'll take too long". Often it's a valid problem but one that can be solved except it shuts down the conversation by making it seem like a brick wall. If you don't immediately know how to solve the problem, it's hard to argue that "My intuition says we can probably find a way round that later".