LFD stands for: least fadded driver. It is a term used to assign the task of driving home from a party considering that each member of the attending group consumed some degree of alcohol. Unlike the "DD" rule, LFD acknowledges that the indiviudals will range, usually, from buzzed to shacked and possibily even demolished; LFD is not applicable if someone is completely sober.
Brandon, Matt, Vince, and Blake all attend an epic party, but need to be home by 12:30. Considering that Brandon is so drunk that he mackin on girls that coud pass as lepers, Matt is lost in his body and arguing with his own vomit, Vince is hookin up with a plant, and Blake is screaming belliegerantly, "CLAM! NOW!", the rule would say that Brandon, is slightly less drunk enough to be the LFD. Even though everyone is just completely totalled, the fact Brandon has only lost his judgement qualifies him for the job.