What adjective describes something that is difficult to believe, likely lacking credibility?

The word "implausible" is the most fitting adjective for describing something that is difficult to believe and likely lacks credibility. This term specifically suggests that something is not reasonable or probable, which directly ties into the idea of skepticism regarding its truthfulness or likelihood.

While "incredible" can suggest something remarkable or hard to believe, it more often has a positive connotation in current usage, particularly in informal contexts where it can imply astonishing or extraordinary events. The word "unlikely" conveys that something is not likely to happen but doesn't necessarily assign a sense of disbelief or skepticism about its credibility. "Dubious," on the other hand, implies a sense of doubt or hesitation, but it carries a more nuanced implication of suspicion or mistrust rather than outright disbelief.

Overall, "implausible" precisely aligns with the notion of something being very hard to accept as true, effectively encompassing both the challenge to belief and the lack of credibility in the assertion.