Seriously scary. To be scared in a serious manner.
It is getting scariously slow at work.
So serious, it's scary. Usually used as an exclamation of shock and disbelief as to the truth/reality of a particular story. Can be used effectively for sarcasm.
"Cindy was at the hair dresser changing her hair from blonde to brown, but the guy knocked red dye into the basin, now she is ginger!" "SCARIOUSLY?"
The reality of this situation is so scary and serious, it's 'Scarious'.