like nailing jelly to a tree
adj. Used to describe a task thought to be impossible, esp. one in which the difficulty arises from poor specification or inherent slipperiness in the problem domain. "Trying to display the `prettiest' arrangement of nodes and arcs that diagrams a given graph is like nailing jelly to a tree, because nobody's sure what `prettiest' means algorithmically."
Hacker use of this term may recall mainstream slang originated early in the 20th century by President Theodore Roosevelt. There is a legend that, weary of inconclusive talks with Colombia over the right to dig a canal through its then-province Panama, he remarked, "Negotiating with those pirates is like trying to nail currant jelly to the wall." Roosevelt's government subsequently encouraged the anti-Colombian insurgency that created the nation of Panama.