Back to it:
Archive: an accumulation of historical records or materials—in any medium—or the physical facility in which they are located. The etymology is straightforward. The word in its modern usage first appeared around 1600. It derives from the French archif, sixteenth century, from Late Latin archivum (plural archiva), “written records” or the place where they are kept, from Greek ta arkheia, “public records.”
Flabbergast: According to Merriam-Webster, the word means to overcome with shock, surprise, or wonder. It lists its origin as “unknown.” Even the Online Etymology Dictionary can only guess at the word’s origin, suggesting that it is an arbitrary formation—somebody just made it up.
Hornswoggle: Merriam-Webster says the word means to bamboozle or hoax. It gives no etymology. The Online Etymology Dictionary says it’s probably a “fanciful formation,” again suggesting that somebody just made it up. Other sources offer various guesses but no firm evidence of origin.
Irk: Merriam-Webster says “irk” means is to irritate or disgust but offers no etymology. The Online Etymology Dictionary says the word comes from the early fifteenth century verb, irken, “to trouble (someone), disturb, hinder, annoy.”
Rare: As a verb, the word means for a horse to rise on two legs. None of the sources I checked could provide a firm etymology. The best guess is that the word comes from the verb rear.
Mordant: According to Merriam-Webster, the word means biting or caustic in thought, manner, or style. It comes from the French word mordre, to bite.
Scofflaw: According to Merriam-Webster, a contemptuous lawbreaker, especially one who ignores parking tickets. It is a made-up word formed by combining the verb scoff with the noun law. The Online Etymology Dictionary says that word originated in 1924. It was the winning entry (from among more than 25,000) in a national contest during Prohibition to coin a word to characterize a person who drinks illegally.
More when the spirit moves me.