Source : Free On-Line Dictionary of Computing

like nailing jelly to a tree
     
         Used to describe a task thought to be
        impossible, especially 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."
     
        [{Jargon File}]
     
        (1997-12-23)