Fuck me backwards!

Jocular expression for surprise, said specially by british people (or at least i haven't heard any american saying it). Quite like "Oh my God"

What it means is FML

an expression of heightened disbelief at an incident or occurrence, similar to fuck me running, but implying even more depth of utter disbelief at that which is becoming known to you at the given moment. Denotes the purest amazement that the truly unthinkable has really just occurred.

used by a guy when he sees a really hot girl

An expression of utter disbelief at how messed up a situation has become. Note there is an ASL sign for this term. With your left hand hold your index and middle fingers down with all other fingers curled up, a sort of upside down peace sign. with your right hand make the same sign with the fingers pointing up, but together. With an arcing swing, bring the right fingers into the "crotch" of the left fingers "legs". Then with both hands still locked, shake them briefly as you move them from left to right, as if the figure was running backwards, with the chainsaw still stuck in its ass.

when you wanna say FML