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"Girl, boo" is used to show disapproval of someone or their comments.

Also meaning that they're wrong, don't fit in or are ugly.

(Especially when a girl says something STUPID or random)
Friend: "Saquisha is going out with Marque."
Me: "No, she's not."
Friend: "Yes, she is."
Me: "Girl, boo."
by ZuRugally August 20, 2010