On the title page of R. Moses Edri's (see here) book תורת חיים there is something which would seem to bring some evidence to the question of the meaning of the abbreviation ס"ט following someone's name.
If you ask the average person, you are likely to hear that it stands for ספרדי טהור, Pure Sepharadi, and either means that the person is of real Sephardic ancestry on both sides, or that they are in fact descended from Spanish exiles, as opposed to Marranos, meaning Sepharadim who remained pure," as opposed to "Pure Sepharadic lineage." Many people have noticed that, at least the first interpretation, is bizarre. While Sepharadi pride was real, is real and can be shown, it would seem that this is going too far. As for the latter, while it might make sense that descendents of Jews who left Spain and Portugal rather than convert would be proud and even willing to lord it over the descendents of those who converted, there are additional problems with these possible meanings.
One such problem concerns the use of the acronym by even non-Sepharadim, such as Chacham Tzvi Ashkenazy and his son R. Jacob Emden.
See below, from H.J. Zimmels Ashkenazim and Sephardim, pp. 286-87:
As you can see, the meaning endorsed here (starting, it seems, with Zunz; see here, and below) is that it stands for סופו טוב, a sort of good wish for one's ultimate fate.
Marc Shapiro dealt with this subject in a typically interesting post at the Seforim Blog. There he basically endorses this interpretation, noting that an alternative meaning that is sometimes given is סין טין, an Aramaism based on Isaiah 57:20 (רפש וטיט). In this interpretation, it is a formulaic expression of the humility, equivalent to prefacing one's own name with הצעיר or הקטן. However, this interpretation is rejected by Zimmels as unlikely (in a note inserted into a revised edition of his book, see below):
However, I propose that this is actually not unlikely in light of the evidence below. This is the title page for the book by R. Edri mentioned in the first paragraph(from 1792):
and the relevant detail:
While Zimmels is right that the ו seems to be missing from the standard ס"ט, here we see what is most certainly the same acronym, only it is spelled סי"ט. Most likely this is, in fact, סין טין. Alternately, one can also admit that it is possible that the typesetter mistakenly placed a י instead of a ו, and really the acronym should read סו"ט. However, I think that is not necessary to go that far, and סי"ט was probably intended. So it doesn't exactly match the verse fragment from Isaiah? So. Neither does it's translation to Aramaic. There's no rule that acronyms must meet some degree of exactitude.