Source : Free On-Line Dictionary of Computing

pessimising compiler
     
        /pes'*-mi:z"ing k*m-pi:l"r/ (Antonym of "{optimising
        compiler}") A {compiler} that produces {object code} that is
        worse than the straightforward or obvious hand translation.
        The implication is that the compiler is actually trying to
        optimise the program, but through excessive cleverness is
        doing the opposite.  A few pessimizing compilers have been
        written on purpose, however, as pranks or burlesques.
     
        (1995-02-01)