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A rhetorical question meant to express that one is indifferent to or finds no importance in what's being discussed. So what if you don't finish high school? Plenty of people drop out and go on to have great lives. John: "You've got a lot of great qualities; any woman would be mad not to think you're a catch!" Dave: "So what? Sarah was the only girl for me, and now she's gone forever."
See also: what