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1.
A word used in substitution for "forgodsake" it is mainly used by those afraid of hellfire because they prefer to never take the lord's name in vain. Of course the holiest of rollers will prefer "forgoshsake" or in a pinch even "forcrapsake" but sooner or later there will come a time when they are so utterly shocked by some dumbass or even some dumbass's elephant that none of the milder forms of this word will suffice.
"FORFUCKSAKE !! will you get your fucking elephant off my fucking foot!!"

"FORFUCKSAKE!! no way am I paying that much for a fucking wheelchair!! I'll just ride the FUCKING ELEPHANT!!
by BoBo WoBo December 30, 2005
 
2.
an expression of annoyance at a situation or someone's stupidity through their words or actions.
pat:"people seem to think its for fuck's sake, mostly americans"

Jerome:"ah for fuck sake"
by Aidsbomb August 19, 2011