A person who frequently spends time at Starbucks, not to get coffee but just to hang out for lack of anything better to do. Similar to the word mallrat, except it derives from the fact that "Starbucks" spelled backwords and minus the C is "Skubrats".
Eric: There's nothing to do around here.
Ronnie: Let's just go chill at Starbucks - but I don't want coffee, I just want to sit outside it and smoke and talk to the other people hanging out there.
Tom: You guys are such skubrats.