We study how the coarseness of moral language affects moral behavior. The conceptual inflation hypothesis and our social signaling model suggest that broadening a negative moral category can have unintended consequences. As a reaction to the new, coarsened moral category, some people may take a less moral action than previously, thereby diluting the category’s meaning. We call this behavioral pattern the diluting effect. Data from an online experiment show that the diluting effect exists. Depending on the shape of the welfare function, the diluting effect makes a narrow negative moral category preferable to a broad negative moral category.