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!