A word either invented by or popularized by the TV show, The OC.
It means to publically insult or demean someone even if it's unintentional, especially when they're around someone they're trying to impress.
similar to cramp your style
perhaps derived from "sult," as in "insult my game."
"Dude, when you said that he was late for his curfew, you totally salted his game."