Loathe as I am to say it, the most logical source for the word ‘nincompoop’ – a colourful word meaning fool or simpleton, is the Wiktionary which posits the following:
Earliest known use of the variation ‘nincompoop’ (presumably influenced by the exigencies of pronunciation, since the trip from ‘i’ to ‘c’ without the slither of an ‘n’ could induce a full-bore glottal stop) is from 1680 – and apparently used in 1694 by Thomas d’Urfey in the Comical History of Don Quixote. I can see why the Latin phrase might well have been agglutinated and sufficiently mangled to become ‘nincompoop’ – another word for which is ‘dunderhead’, also invented in the seventeenth century, possibly from the Dutch ‘donder’ for thunder, but why add this to the English ‘head’? The trusty Online Etymology Dictionary concurs with this origin, and points to ‘blunderbuss’ as springing from the same Dutch root – ‘donder’ – plus ‘bus’, for gun – and a blunderbuss (cfr. Jack White for Blunderbuss, an excellent foray into the thunderous and wondrous) is a gun, so that makes sense.
Rapscallion seems to be a purely linguistic adaptation from ‘rascallion’, from French ‘rascaille’ (a very low-order of French soldier), translated into English rascal, and all from ‘rash’ meaning impetuous. A rapscallion, of course, is a rogue and a scalawag, familiar from movies about gambling and riverboats.
For scalawag, the OED (see how they did that? I mean the Online Etymology Dictionary, and not the revered and real OED) says the following:
“disreputable fellow,” 1848, American English, originally in trade union jargon, of uncertain origin, perhaps an alteration of Scottish scallag “farm servant, rustic” (by influence ofwag “habitual joker”). An early recorded sense was “undersized or worthless animal” (1854), which suggests an alteration of Scalloway, one of the Shetland Islands, in reference to little Shetland ponies. In U.S. history, used from 1862 of anti-Confederate native white Southerners.”
Scalawag and rapscallion are both associated with the American South, and therefore their reference to disreputable types was firmly grounded in the political. Nincompoops, however, are politically neutral.