A troublesome situation; an issue whose resolution is difficult or contentious but not necessarily complex.
A complex, troublesome situation arising when a decision or action produces considerable subsequent problems.
Used other than figuratively or idiomatically: see can, worm.
A situation, person, group, etc. which is difficult to overcome or deal with.
A problem that is challenging to solve.
Used other than figuratively or idiomatically: see hard, nut, crack.
A place, opportunity, etc. to which it is difficult to gain entry.
An amount that is difficult to finance.