1. To have not the slightest idea of what something means, especially when you think that no one else would understand either because the information is confused or incomprehensible.
2. Of deaf or hearing impaired people, to be completely unable to follow someone's speech (either through listening or lip reading).
3. To be so surprised at something that you have no idea what to say. English = (be) speechless, (be) dumbfounded.