A 'nincompoop' is, according to many Googled online sources, a 'silly, stupid or foolish person.' The root meaning, or 'origin,' or 'etymology' of the word is not really known (at least, not by anyone I could find in a short search :). However, for a really enjoyable journey into how words came to be, go to the Take Our Word For It website see link
One theory is that is is a corruption of 'non compos mentis' - Latin for 'not of sound mind'.
"nimcompook" does not appear to be a recognized term or concept. It is possible that it is a made-up or misspelled word. If you have more context or details about what nimcompook refers to, please provide them so I can help clarify.