Whaaa? is a running gag related to a reaction usually made by Chris Griffin. The gag is usually used when Chris finds out something shocking, reacts as if it's no big deal and then shouts "Whaaaaaaaaaaaaaaaaaa?"
Chris finds out that he can never see his father again. He says, "That's okay, he's very—Whaaaaaaaaaaa?"
Chris finds out that because he made the whole tribe dance, he is to be married and he says, "Well, if you're happy then I'm happy, because after all — Whaaaaaaaaaaaaaaa?"
Peter walks in on Lois and Bill Clinton after sex, then Peter says, "Hey, Bill, you up for a little bowling? I swiped some money from Lois' purse, but I don't think she'll notice because she's here-- HUMPING YOU??!!!"
When Lois starts to introduce a jealous Peter to an old college flame, Peter responds with "If he tries anything I'm going to kick him right in the balllllll---gina?" When he sees it's Lois' female friend Naomi.