Sometimes I have thought SLF to be derogatory, but then I remember this is a Professional pilots website. Like many industries, staff have nicknames for customers, so it is no surprise to find it in the aviation industry, which is second only to the world of computing for the adoption of acronyms and non standard descriptions.
As has been rightly pointed out, it is only a problems when some airline staff take it (stupidly) literally. Fortunately most airline staff are wise to the fact that their job depends on 'SLF' or 'paying customers' in the real world.
The term Self loading freight is actually marginally humorous when you think about it.