Implausible means difficult or impossible to believe because it seems unlikely or unrealistic. When something is implausible, it might be technically possible, but it strains credibility. If your friend claims their dog ate their homework, drove to school, and turned it in personally, that's an implausible excuse. Could a dog theoretically do those things? Maybe in some bizarre universe, but the story is so unlikely that no reasonable person would believe it.

Implausible is stronger than just “unlikely.” An unlikely event might happen occasionally, like rolling six sixes in a row with dice. An implausible explanation makes you raise your eyebrows and think, “Come on, really?” In mysteries and detective stories, investigators often reject implausible theories, even if they can't immediately prove them wrong, because they simply don't make sense given what they know.

Writers sometimes create implausible plots where too many coincidences happen or characters act in ways that don't fit their personalities. Readers notice these implausible moments and find them frustrating because good stories feel believable even when they're fictional.

The opposite is plausible, meaning believable or reasonable. A plausible excuse might not be true, but at least it could be true without stretching your imagination too far.