Is this from the radio edit of the Jay-Z song "Can I get a...?" If so, "what-what" is just what they inserted into the clean version of the song to replace a phrase that has curse words that can't be said on the radio. I can't write it here, but you can easily find the lyrics to the dirty version if you search the web. Otherwise, it might be something someone says when they can't remember the name of the thing they're asking for. In that case, they might also use other silly words like "thingamajig" or "whatchamacallit."