1) A word to replace 'Oh my god', or 'omg', and so on, or to express your emotions (shock and horror, duuh)
2) Can also be used when surprised.
Teacher: And... Stacy, you seem to have broken your record of always getting the bottom marks. Well done, you got 100% on your maths paper.
Stacy: Shock horror, say what?!
Rapee: SHOCK HORROR!