From Longman Dictionary of Contemporary Englishslack-jawedˈslack-jawed adjective having your mouth slightly open, especially because you are surprised or stupid SYN open-mouthed They looked at him, slack-jawed with disbelief.
Examples from the Corpus
slack-jawed• The inmates are stunned in slack-jawed amazement.• She gasped repeatedly and Creed, goggle-eyed and slack-jawed, panted in time with her gasping.• He looked out with an expression not yet of terror but of total, slack-jawed surprise.• The audience sat slack-jawed with disbelief. 