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!