We were born helpless. Then they named you, gave you a religion, a nationality, and imprisoned you. You then live/strive all your life to live up to the labels they placed on you. HOW STRANGE!
Essentially, a nincompoop is a fool, a stupid person whose incompetence or naïveté is quite apparent. It is often used to describe people of a much lower social ranking, although some may describe an unpopular or incompetent leader as an "utter nincompoop." It is conceivable that the originator of "nincompoop" was seeking to combine several unsophisticated words into one nonsensical but descriptive word.