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?"
of this situation is so scary and serious, it's 'Scarious'.