Has to do with a male that is having sex, and that night he gets one loose girl and is lucky enough to fist her, because most girls do not allow guys to fist them so when they do, you're lucky.
Lucky = Luc ..Fisting = Fist
Oh man, tarc got so lucky last night he got to lucfist that hot older mom we saw walk in.
LucFist is the act of a boy/male/man getting lucky (luc) and being able to fist a girl, since girls do not normally allow that to happen, it does not necessarily constitute having sex, it could just be an event/situation of fisting that occurs between the man and the woman.
A person who lucfists or is known as a "lucfister" has a notorious reputation for being "the man" because most guys never succeed in being able to fist a girl.
Holy crap man, Steve and Lucas are notorious for the lucfist, no one scores those kind of women like they do.