a) One is in a difficult situation.
b) Has heard something unbelieveable or shocking.
c) Has hurt themselves.
An extended Australian version: "Fuck me sideways with a Yabbi Pump!" (Yabbi Pump: Suction device used to pump Yabbis from the sand)
a) "Fuck me sideways, wrong hole!"
b) "I just won a million dollars!" "Well fuck me sideways!"
c) (*Slams toe into wall*) "Fuck me sideways that hurt!"
"I just rooted a Kangaroo!" "Fuck me sideways with a Yabbi Pump!"