"Out-of-this-world" is an idiom that means unbelievable, fantastic, or extraordinary. If you are looking for synonyms that could replace this term, you could use "amazing," "astounding," "phenomenal," "incredible," "unbelievable," "mind-blowing," "mind-boggling," "spectacular," "jaw-dropping," "awe-inspiring," or "unfathomable." These words can be used to describe anything or anybody that is out of the ordinary or exceeds expectations. Synonyms for "out-of-this-world" are useful in writing and conversational language when you want to emphasize your admiration for something or someone.