The phrase "unworthy of belief" is often used to express disbelief or a lack of trust in something. There are many synonyms for this phrase that can be used to convey a similar meaning. Some common synonyms include "incredible," "implausible," "dubious," "questionable," "unreliable," "untrustworthy," and "doubtful." Other synonyms might include "fishy," "unconvincing," "fantastic," "nonsensical," "unfounded," or "unsupported." Each of these words carries a slightly different connotation and can be used to describe different levels of disbelief or skepticism, depending on the context in which they are used. Ultimately, all of these synonyms express the same central idea: something is not to be believed.