When the words "terrified" and "petrified" is just is not enough to express how horrified you really are.
A combination of "terrify" and "petrify".
Simply unbelievably pee your pants, shivers down your spine, goosebumps, scary.
When the clown jumped out at Shanaynay at the haunted house, she was truly tetrified; she had always been scared of clowns since she watched It.
That movie was tetrifying. I literally peed my pants when the donkey ate that girl!