Were you aware that the "farious" in "multifarious" and "nefarious" come from the same root? 'Streuth! Viz., far, the Latin word for spelt/grain/meal/etc.. Grain supplies were of course an important matter in Rome (consider Gracchus' corn laws), and "multifarious" was a term of envy, or praise, or, as is not unheard of, both: of someone who had both much grain and many varieties thereof, spelt and barley and wheat and whatnot, one might say that he was multifarious; with time the emphasis on plenty gave way to that on variety, and it began to be used in what was at first sensu lato but has become by our day sensu stricto. The history of "nefarious", "no-grain", is even more closely linked to the importance of grain to Roman society; indeed, similar words with similar histories are attested in nearly all farming cultures. Among the harvesters it was customary to burn a few sheafs as a sort of sacrifice to the harvest deities, and there were two reasons that someone might not pursue this custom: either because he had nothing to burn, or because he deliberately spurned it (of course one might deliberately spurn it and therefore have saved nothing, but you know what I mean). In the former case one was held to have been disfavored by the deity; in the latter, certainly to be about to incur the disfavor. In either case the epithet "nefarius" was applied to the unhappy soul. As with "multifarious" the word underwent an expansion in meaning, though in this case it wasn't quite as general, for its expansion concerned for a long time merely the number of deities whose disfavor might earn one the title of nefas, which increased to include the entire pantheon, regardless of how the god or goddess felt about grain. By our time, of course, it means anything sinister, immoral, or impious.
"Ambiguous" is both of much more recent vintage and wholly unrelated to the above, but I'll tell you about its coinage here anyway: it comes from Belgium, of all places, in the period of time when brewers were experimenting with different ways of bottling fruitless lambics. At that time anything which underwent a secondary fermentation in the bottle would be called "gueuze", regardless of its composition—that is, regardless of whether the lambic in the bottle was "old" or "young". Some brewers and bottlers, realizing that a more complex and interesting flavor could be had by mixing old and young lambics, marketed it to the upper classes, attempting to appeal to their sophistication, with a Greek prefix: an ambi-gueuze, that is, gueuze made with both. Though this style of beer never caught on in England, the word did drift over, signifying anything in which two elements were mixed, not cleanly again to be recovered, and, after the spelling and pronunciation were suitably Angularized, became our "ambiguous".