intractable

Short definition from Wiktionary

Adjective

1. 1
   Not tractable; not able to be managed, controlled, governed or directed.
2. 2
   (of a mathematical problem) Not able to be solved in polynomial time; too difficult to attempt to solve.
3. 3
   Difficult to deal with, solve, or manage. (of a problem)
4. 4
   Stubborn; obstinate. (of a person)
5. 5
   Difficult to treat (of a medical condition).