The word "brassing" typically refers to the discoloration or tarnishing of metal, particularly brass. Some synonyms for this word could include oxidation, corrosion, rusting, or patina. These are all related to the chemical or physical processes that can cause metal to change color or texture over time. Another way to describe the same phenomenon might be to use phrases like "turning green," "becoming dull," or "losing its shine." Depending on the context, there may be additional terms that could be used to describe this type of metal deterioration, such as weathering, erosion, or wear and tear.