[allegedly from a World War II military term meaning "ten pounds of manure in a five-pound bag"]
n. 1. An intractable problem.
2. A crucial piece of hardware that can't be fixed or replaced if it breaks.
3. A tool that has been hacked over by so many incompetent programmers that it has become an unmaintainable tissue of hacks.
4. An out-of-control but unkillable development effort.
5. An embarrassing bug that pops up during a customer demo.
This term has other meanings in other technical cultures; among experimental physicists and hardware engineers of various kinds it seems to mean any random object of unknown purpose (similar to hackish use of frob).
It has also been used to describe an amusing trick-the-eye drawing resembling a three-pronged fork that appears to depict a three-dimensional object until one realizes that the parts fit together in an impossible way.