frank (adj.)
c.1300, "free, liberal, generous," from Old French franc "free (not servile), sincere, genuine, open, gracious; worthy" (12c.), from Medieval Latin Franc "a freeman, a Frank" (see Frank). The connection is that only Franks, as the conquering class, had the status of freemen. Sense of "outspoken" first recorded in English 1540s.
Frank
one of the Germanic people that conquered Celtic northern Gaul from the Romans c.500 C.E. and made it into France, from Frankish *Frank (cognate with Old High German Franko, Old English Franca). The origin of the ethnic name is uncertain; it traditionally is said to be from the old Germanic word *frankon "javelin, lance" (compare Old English franca), their preferred weapon, but the reverse may be the case. Compare also Saxon, traditionally from root of Old English seax "knife." In the Levant, this was the name given to anyone of Western nationality (compare Feringhee and lingua franca).
frank (n.)
short for frankfurter, by 1916, American English. Franks and beans attested by 1953.
frank (v.)
"to free a letter for carriage or an article for publication," 1708, from shortened form of French affranchir, from the same source as frank (adj.). Related: Franked; franking.