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.

[glossary]
[Reference(s) to this entry by made by: {nailing jelly}]