Plausible is an adjective that means "believable" or "credible." Therefore, its antonym, unplausible, would be an adjective that means "not believable" or "not credible." Some antonyms for unplausible include believable, credible, likely, reasonable, realistic, and plausible itself. If something is described as unplausible, it is generally considered to be improbable or unlikely to happen. It could be argued that opinions, rumors, or conspiracy theories that lack evidence or rationale are often described as unplausible. In contrast, scientific research, logical arguments, or factual evidence are usually viewed as plausible, and therefore, believable.