There are numerous synonyms for the word "impugnable" that can be used to express the same idea or concept. Some of these synonyms include undefendable, challengeable, assailable, contestable, vulnerable, disputable, and criticizable. Each of these words can convey a slightly different shade of meaning, but they all suggest the same underlying notion of being open to attack or criticism. In addition to these synonyms, there may be other words or phrases that can be used in context to convey a similar sense of vulnerability or susceptibility to critique. By using a variety of synonyms for "impugnable," writers can ensure that they are communicating their ideas with maximum clarity and precision.