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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.