An acronym standing for "self-aggrandizing little sh*ts". This is a way to refer to (usually young) people who spend time telling others how great they are for relatively mundane accomplishments or who demand a level of respect that they haven't earned. Can be used to describe an individual or a group. "Sallies" can used as a feminine modifier.
"Those guys at table 12 sure are sals. This is Denny's, for crying out loud. If they snap their fingers at me one more time, they are getting the special sauce."