Stands for "For Crying Out Loud", The original phrase being popularized by the television show Stargate. Common to online first-person shooters. Intended to parody other acronyms such as FFS.
InternetNitwit1: "FFS, leran to play ur class noob!"
Further slangification of "p.o", an abbreviation of "Post Office".
"I'm going to check the po to see if I've got any college mail."