Comment by GrumpyYoungMan

"reversing" in this sense is shorthand for "reverse engineering" or the analysis and reconstruction of the code for a executable binary at the assembly language level without the benefit of the original source code or debug symbols. The most common legitimate use is for analyzing malware and viruses. The best known product in this field is IDA Pro. (It would have been nice for the site to provide a comparison between the two, since such a comparison is inevitable anyway.)