As a Verb or Adjective:
1. To be so surprised by what someone says or does that you cannot think of anything to say or do. English = (be) speechless, (be) flabbergasted, (be) dumbfounded, (be) open-mouthed.
2. To open your mouth in surprise or amazement. English = gape.
3. Of a mouth, to be held wide open in surprise. English = (be) agape.
1. Used alone to mean that you are completely amazed at what someone has said or done such that you do not know what to say or do. English = 'I'm speechless!'