Until today, the pressing issue of how these two utterances differ subtly in meaning never crossed my mind. But upon hearing one of these this morning and realizing that my brain was expecting the other, I was forced to do a thoroughly worthless analysis and post my conclusions in the most irrelevance-friendly site on the entire World Wide Web.
“Uh-uh” in response to a question translates so a simple “no” answer. “Nuh-uh,” on the other hand, is used not so much as a reply to a query but as a challenge or refutation, in the spirit of “I disagree,” “Gonna call BS on that one” or “Fuck that noise!”
“Uh-uh,” however, enjoys a purpose that its slightly longer sibling does not: It may function as an imperative verb. For example, when snaking your hand up your girl’s inner thigh for the first time and hearing her say “uh-uh,” you know you’re being told to cease and desist. But were she to declare “nuh-uh” instead, you might not be dissuaded in the least and may even think that she’s encouraging you but telling you to aim for a sightly more sensitive spot.
By the way, I’ve anticipated all of you who have already made plans to respond to this with an “uh-uh” (if trying to be ironic on two levels) or a “nuh-uh” (if shooting only for one). Understand that this is a no-no.