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Well, I found this:

"S.S.T." is a song by Prince which was recorded and released directly after the impact of 2005's Hurricane Katrina. It was officially released as a digital download by the NPG Music Club on September 3, 2005 and reached number one on the iTunes R&B chart. It was later made available as a CD single through commercial outlets. All proceeds from the recording went to hurricane relief.

The song's title alludes to Sade's 1985 song "The Sweetest Taboo", which is namechecked more than once in the lyrics. "S.S.T." also refers to sea surface temperature, which is used to monitor the threat of hurricanes. http://en.wikipedia.org/wiki/S.S.T.

Or it could refer to anything on this list http://en.wikipedia.org/wiki/SST 

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