"Sheer disbelief" means a feeling of complete and utter astonishment or shock, when something is so surprising or unbelievable that it's hard to accept or comprehend. Full definition
If I laughed or smiled at all it was in sheer disbelief of what I was witnessing. (lights-camera-jackson.com)
Sheffner expressed sheer disbelief at the possibility that an attorney who blogs as a hobby and has no official involvement in a matter could be liable for violating ethics rules by offering analysis in an ongoing matter. (legalblogwatch.typepad.com)
To this day, I don't know whether to laugh, to cry or hide my head in embarassment and sheer disbelief. (everydayfamily.com)