It stands for "subtle slap to face"; A moment when someone insults someone in public in a way that is evident to everyone else except for the targeted person.
If a group of people have been talking about how poor a person is, and then the person joins the group. One of the people in the group might say something like "nice shoes where did you get them from" knowing good and well that the shoes are of low quality. The person who said the rude comment just used the SSTF method.
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