An acronym for "For fuck's sake", meaning "I can't believe this
shit, I demand you to stop"
The source is For Christ's sake ("in Christ's name please explain/stop that"). Swap in Fuck All ("nothing") for the deity, and you
get an urban, angry, non-religious, expletive, demanding equivalent of "Oh My God, do
something".
1. For fuck's
sake, where did you get that godawful
haircut?
2. FFS, I've explained it to you five times already. Now
STFU!