If you’ve never heard the expression, it refers to the practice of carrying programming habits to a new context, even when they are inappropriate. Some programmers who began with FORTRAN continued to write programs that strongly resembled FORTRAN after they had moved to other languages. They remained too set in their ways and didn’t adapt their programming approach to a new way.
(I should point out that I always liked FORTRAN. Many of the failings people ascribe to FORTRAN-77 are related more to bad programming than the language itself. Also, Fortran-90 later addressed most of the actual weaknesses of FORTRAN-77.)