Allow assignment to constant arguments.
Allow a subprogram to change a dummy argument that is a constant. This option is provided only to allow legacy code to compile and execute without a runtime error.
Without -copyargs, if you pass a constant argument to a subroutine, and then within the subroutine try to change that constant, the run aborts.
With -copyargs, if you pass a constant argument to a subroutine, and then within the subroutine change that constant, the run does not necessarily abort.
Code that aborts unless compiled with -copyargs is, of course, not Fortran standard compliant. Also, such code is often unpredictable.