The fucking *Britannica*, when presented with both variants, says, A) it's sorry, but the [link|http://www.britannica.com/search?query=numskull&submit=Find&source=MWTEXT|result for "numskull"] is only for paying customers(*) -- and in stark contrat thereto, B) claims it's [link|http://www.britannica.com/search?query=numbskull&submit=Find&source=MWTEXT|unable to find any result for "numbskull"]!
Oh well, I take at least some consolation from the fact that you seem just as surprised and sceptical about this as I was.
(*): I *hate* that smarmy way all too many businesses -- from gyms and video-rentals to arseholes lie these Britannica types -- have, of calling their customers "members". Don't you?