Quite frankly is a popular colloquial phrase, used in all sorts of contexts by quite literally everyone.
It is used for highlighting the fact that the opinions following the phrase are completely sincere and honest.
Its modern-day slang equivalent is FR, or For Real.
“Quite frankly” is a construction, describing a statement as very frank, or very honest.
Frank itself came into the English language around the 14th century, with an initial meaning of “free, liberal, generous”, originating from the tribal name of the Franks, who were the sole freemen of modern-day France, following their conquest of Gaul during the 5th century.
The meaning of the word Frank shifted toward honesty in the 15th and 16th centuries, and one of the earliest documented cases of the phrase “Quite frankly” comes from the 1623 “Calendar of State Papers and Manuscripts, Relating to English Affairs Existing in the Archives and Collection of Venice, and in Other Libraries of Northern Italy”.
In the subsequent centuries, the phrase “Quite frankly” didn’t quite frankly see the widespread use, one would expect it to.
Its appearances in print started rising in the second half of the 1800’s, which further escalated at the turn of the 1900’s.
Throughout the 20th century, it experienced widespread use, appearing in literally all sorts of contexts, from everyday conversations to cultural products, such as music, literature and movies.