As a Noun:
1. The feeling of being very surprised about something, especially when this is shown by opening your mouth wide. English = surprise, amazement.
2. The feeling of being so surpassed about something that you don't know what to say or do.
As a Verb or Adjective:
1. To be very surprised about something. People often open their mouth wide when they feel this way. English = (be) open-mouthed, (be) agape, (be) flabbergasted.
2. To be open-mouthed and unable to speak because you are so surprised. English = (be) speechless, (be) dumbfounded.