SC (lower case with a period): the abbreviation of scilicet, which in academic writing introduces a clarification by further example.
SD - the United States’ State of South Dakota
SE – Standard English - the national norm in an English-speaking country, especially as the language for public and formal usage.
SF = Science Fiction, of course.
SG - The Gibson Standard (or Special) Guitar
SH - Sacrifice hit, in baseball scoring
SI - the International System of Units, from French Système international
I once lived in San Jose CA. From what I heard, it is not uncommon for people who think they’re getting on a plane to San Jose, Costa Rica, to wind up in California, and vice versa.
SK - a tool company based in Illinois
SL - Sharia law, a Muslim legal system
SM - Sadomasochism. Wiki: “The abbreviation S&M is often used for sadomasochism, although practitioners themselves normally remove the ampersand and use the acronym S-M or SM or S/M when written throughout the literature. … The diagnostic guidelines for the ICD-10 [manual of psychiatric illnesses] state that this class of diagnosis should only be made ‘if sadomasochistic activity is the most important source of stimulation or necessary for sexual gratification.’”
SN - Sporting News
SO - significant other. Used when the writer does not wish to specify exact what the relationship is, or when terms like “husband,” “lover,” etc. do not fit the situation.
SP - Navy Shore Patrol, sort of like cops, but usually smarter and less abusive, mostly just there to preserve order.
SQ - SQ (aka Squeeze) compression program. SQ (program) - Wikipedia