Quite frankly, it’s not immediately clear why “frankly” should mean open and honest. It has nothing to do with a truthy guy named Frank, after all.
The word arrived in Middle English somewhere near 1300 from the French “franc,” which at the time did not yet have to do with monetary currency. Instead, in both its French and English editions, it meant “free.”
“Franc”, the source of “frank,” came from “francus,” a Latin word that meant free, but also referred to the tribes that gave France its name; the Franks. The Franks, in turn, MIGHT have been called “Franks” because their signature weapon was a kind of javelin that was called a “frankon” in Germanic languages. Nobody seems to be 100% sure of that. But anyway, back to the “free” meaning. After the Franks took over Gaul (which is what France was called before the Franks arrived), political freedom was granted only to themselves and to anybody they specifically designated as under their protection. So by the time the word made it to English, “franc” had come to mean “free” because of that political arrangement.
It’s also not 100% clear, by the way, whether “franc” meant “free” before or after it referred to the Franks. There’s some evidence that it meant “free” before it meant an ethnic group, but there’s also evidence on the other side.
After “frank” arrived in English, it took a couple of centuries for the meaning to evolve from “free” to “generous,” “open,” and finally in the 1500s, “candid.” And there’s it’s been ever since. As for “quite frankly,” the phrase that means “even more frank,” it arrived in the 1800s when English speakers started to use “quite” to intensify all sorts of words. The earliest example of “quite frankly” as a “sentence adverb” — that is, a starting adverb that modifies the whole following sentence, not just a single word — is from a Valentine poem published in Punch magazine in 1894:
“…Quite frankly I wouldn’t be thought to defend it
(Though I swear that I bought it as perfectly new);
And the reason, in fact, why I happen to send it,
Is to have an excuse for a letter—to you.”
The poem was published anonymously, which, frankly, was probably a wise choice.
