A word used by the friends and the members of the band The Academy Is...
"Santi" was the shortened version of "Santiago," the last name of a boy that Adam T. Siska
(bassist for the band), knew and strongly disliked in high school. Perhaps "santi" was initially used as a way to refer to Santiago derisively, but eventually the band began to use it as a greeting, a word of praise, or an expression of anything good.
Fans made Adam about a million shirts that said "Santi" on it for a while, which he was delighted about.
"Santi" was also tatooed on Andy Mrotek
aka Butcher, the drummer for the band. You can see it for a moment in the video of the band's song "Slow Down." The tatoo was located on his butt before he replaced it with a different one.