Short for Lost in Space Fan. A fan of the 1960s science fiction TV series Lost in Space.
This is also spelled as LISFAN, Lisfan, LIS fan, LISfan, LISFan, and so on.
This is also the name of the foremost fan magazine (or fanzine) for Lost in Space, which was published from 1981 to 1992. Lisfan is no longer published, but its publishing company Lisfan Press, will be coming out with new books in 2005.
He is a not a Star Trek fan, he is a LISfan.