An initialism for Secret Master of Fandom, smof has come to be more-or-less a generic noun for a perennial conrunner — in particular, a Worldcon runner, or a fan who is a regular at the WSFS Business Meeting. As a verb, to smof means to talk in a sercon manner about running conventions, other aspects of fandom or fan politics. (It's also sometimes used casually to describe intense, geeky discussion on any subject, e.g. "You don't want to go in there, they're smoffing crochet patterns.")
"The Secret Masters of Fandom" began as a tongue-in-cheek reference to the mythical group of fans who supposedly convened in smoke-filled back rooms to "really" decide the course of future fandom, a jibe at the notion that the anarchistic meritocracy that is fandom is controlled by some powerful, behind-the-scenes cadre of fans. At Discon 2, Jodie Offutt once remarked that the real Secret Masters of Fandom were the fen who kept con-hotel bathtubs filled with ice and bheer.
With the explosion of conventions in the 1970s, it became taken up as the name for a loosely organized group of experienced convention problem-solvers — former Worldcon or major convention chairs of note, gonzo hotel negotiators, noted con programming types, etc. — who got together from time to time to engage in a little timebinding, pass on (or volunteer) their experience, and thereby ensure that fandom gets to do what fandom wants to do. (It now takes more fans to run a Worldcon than once attended them.) This definition was helped along by the creation of the smofs list, a conrunners email list, and later by the creation of Smofcon.
Bruce Pelz once announced that he was Smof #2 — leaving it to others to fight out who is #1.
The initialism was likely coined in conversation, probably by Jack Chalker in 1963, based on a 1950s Bob Tucker article in Quandry. Tucker apparrently derived it from Gerald Kersh's early 1950s book The Secret Masters. The term was used in a skit at the 1963 Worldcon that featured Hal Clement, and a "SMOF Award" was presented to Ted Sturgeon (Forry Ackerman received the award on behalf of the absent Sturgeon) making Sturgeon "whether he likes it or not, an Honorary Member of SMOF."
See also: smofs list, SBOFs.