Leave openmouthed is an idiom that means to astonish, surprise or shock someone to the extent that they are left speechless or with their mouth open. The antonyms for this phrase could include words that describe a lack of surprise or something that is not astonishing. Some of these potential antonyms might include terms like "predictable," "ordinary," "expected," "boring," "unremarkable," or "unsurprising." Additionally, words that indicate a lack of interest or a feeling of indifference might also qualify as antonyms, such as "disinterested," "unimpressed" or "apathetic." On the other hand, words like "calm" or "composed" could also be considered as antonyms, suggesting a level-headed demeanor in the face of potentially shocking or surprising information.