From Longman Dictionary of Contemporary Englishagapea‧gape /əˈɡeɪp/ adjective [not before noun] SURPRISEDwith your mouth wide open, especially because you are surprised or shocked Vince watched, his mouth agape in horror.
Examples from the Corpus
agape• I froze, mouth trout-like, agape in anticipation, heart racing.• She sat there with her mouth agape, staring at the ring.mouth agape• His grandmother watched, her toothless mouth agape.• He left mouths agape not only to the very end, but five days after he was gone. 