It basically just means so hard
Indeed:
Correct me if I'm wrong but (and I hate to link to wiki, but it's the best explanation of polynomial time for a layman (such as me) I could find):
An algorithm is said to be of polynomial time if its running time is upper bounded by a polynomial expression in the size of the input for the algorithm, that is, T(n) = O(nk) for some positive constant k.[1][13] Problems for which a deterministic polynomial-time algorithm exists belong to the complexity class P, which is central in the field of computational complexity theory. Cobham's thesis states that polynomial time is a synonym for "tractable", "feasible", "efficient", or "fast".[14]
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https://en.wikipedia.org/wiki/Time_complexity#Polynomial_time
Thus, NP-Hard problems is those that are
untractable, unfeasible, inefficient and slow. In technical terms
having a polynomial-time reduction. The halting problem is a much used example I think.