Something that's implausible is farfetched or unlikely. If it's 3 pm and you still have to study for three exams and write an essay before midnight, it’s implausible that you’ll also have time to watch a movie.
The adjective implausible breaks down into im, meaning “not,” and plausible, meaning "likely." So it simply means "not likely." Implausible ideas or stories usually get high marks for creativity, but they're just too crazy to be believable. But as philosopher René Descartes noted, “One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.”