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.