Antonyms for the phrase "leaving open-mouthed" could include phrases such as "boring," "mundane," or "expected." These terms suggest that whatever was being witnessed or experienced did not elicit a strong emotional response or surprise. Alternative phrases could also include "unremarkable," "uneventful," or "monotonous," all of which also imply a lack of excitement or interest. In contrast, synonyms for the phrase "leaving open-mouthed" could include descriptions such as "astonishing," "amazing," "incredible," or "mind-blowing," which suggest that the experience or event was truly awe-inspiring and unforgettable.