SLF is a phrase that goes back to PPRuNe's origins. The forums were originally started as a bulletin board for Flight Crew only. A gradual expansion occurred during which this forum was started. SLF was (at that time) a phrase in occasional use by Pilots, particularly those previously flying freight - hence the forum title. I have never heard the phrase used by Cabin Crew, except within the confines of PPRuNe, where it tends to be used for simplicity, as a common term of reference. Truth be told, I've rarely heard the term used by Pilots either, outside of here. The phrase was never intended to be abusive, and I'd have to say that if you find it to be so, your skin is rather thin. The phrase is a form of jargon, much as is some of the language and phraseology used between medical staff - a certain kind of black humour that does not translate well to the uninitiated. I'm sure that some of you will choose to take offence - quite possibly the same people that appear to loathe flying, airlines and their staff in all forms. It is a genuine mystery why those who feel this way should regularly post here, or why, for example (there are others) a simple request for a cardboard boarding card at the door can spin out to multiple pages of vitriol. Still, I suppose that as long as you're polite to each other, such threads can roll on.
Window Shade Blinds (again) - At the risk of banging an old drum, there is a link to the multiple previous discussions on this subject in the forum FAQ. This might possibly be a good place to start.