It is acceptable for SFD to be used spontaneously in courtship by a suitor expressing his/her approval of a desired lover's physical prowess.
Jim: Touchdown South Carolina.
Caroline: Shit. Fuck. Damn.
Jim: The preferred term is SFD.
Jim: They just scored again.
Ronald: Yo Benj, here comes that shawty...
<shawty walks by>
It can be used when your friend makes a moronic comment and their best luck is to just stop where they are at.
Erin: "Simon folows dinosaurs."
Hottie: SFD, Again.
A syndrome in which the said subject has trouble deciphering which thoughts to express aloud, and which to keep silent.
Often the source of embarrassment of subject's companions.
Kyle - Chad has SFD.
Bailey - You reckon so?
Kyle - Yes, he just asked me if I would graffiti the school's walls with him while we were eating thanksgiving dinner at my grandma's house!
Bailey - Yeah... he does have difficulty gauging what to and not to say.
Chad - (rather loudly) GUYS I'M SO DRUNK!!!
Krystan - Chad! Stop being to SFD! I'm on the phone with my mother!!
Chad - I really need to take a shit.
Teacher - ... SFD...
Jade - Well today there was a 40% chance of SFD.
Arch - Nice, better than up here! No a SFD in sight!
Jade - Well ain't that an SFD if I ever did hear one.