From Longman Dictionary of Contemporary Englishsomebody’s jaw droppedsomebody’s jaw droppedused to say that someone was very surprised → drop
Examples from the Corpus
somebody’s jaw dropped• Conley's jaw dropped, and she left without saying anything.somebody’s jaw droppedsomebody’s jaw droppedSURPRISEDSHOCKused to say that someone looked surprised or shocked ‘You’re not serious, are you?’ Ellen’s jaw dropped. → jaw 