intractable

adj

1. Not tractable; not able to be managed, controlled, governed or directed.

   “And I cannot but expect that this will repeatedly lead to the discovery that an initially intractable problem can be factored after all.”

2. Not able to be solved in polynomial time; too difficult to attempt to solve.
3. Difficult to deal with, solve, or manage. (of a problem)

   “Work—bureaucratic work in particular—poses a series of intractable dilemmas that often demand compromises with traditional moral beliefs.”

4. Stubborn; obstinate. (of a person)
5. Difficult to treat (of a medical condition).