Ok Raggy old bean !! Never one to let a mate down.
Up strides the kindly ossonflags with his serious head on to give you the true -if slightly boring - explanation of "BS"

It means "Breeze shooting" as in:

" A bunch of the boys were stood around the old cracker barrel in Zekes store,telling tall tales, scratching their balls and whittling.When in minced the Revr'nd Mcgillucuddy who lisped "Hi Guys you shooting the breeze?" I know, I know!! it is American granted, but we all know it is a euphonism for "BULLSHIT!!!!!"