I stumbled across this nice, clear explanation in Boswell’s Life of Johnson today (p.416 in my Everyman edition):
We talked of languages, Johnson observed that Leibnitz had made some progress in a work, tracing all languages up to the Hebrew: “Why, Sir, (said he,) you would not imagine that the French jour, day, is derived from the Latin dies and yet nothing is more certain; and the intermediate steps are very clear. From dies, comes diurnus. Diu is, by inaccurate ears, or inaccurate pronounciation, easily confounded with giu; then the Italians form a substantive of the ablative of an adjective, and thence giurno, or, as they make it giorno: which is readily contracted into giour, or jour.” He observed that the Bohemian language was true Sclavonick. JOHNSON. “Why, Sir, to be sure, such parts of Sclavonia as confine with Germany, will borrow German words; and such parts as confine with Tartary will borrow Tartar words.”
I doubt that every derivation is quite as clear or easily demonstrated. Linguistics is almost made into a word game here!