lingua franca n.

c. 1300, "free, liberal, generous;" 1540s, "outspoken," from Old French franc "free (not servile); without hindrance, exempt from; sincere, genuine, open, gracious, generous; worthy, noble, illustrious" (12c.), from Medieval Latin francus "free, at liberty, exempt from service," as a noun, "a freeman, a Frank" (see Frank).

Frank, literally, free; the freedom may be in regard to one's own opinions, which is the same as openness, or in regard to things belonging to others, where the freedom may go so far as to be unpleasant, or it may disregard conventional ideas as to reticence. Hence, while openness is consistent with timidity, frankness implies some degree of boldness. [Century Dictionary]

A generalization of the tribal name; the connection is that Franks, as the conquering class, alone had the status of freemen in a world that knew only free, captive, or slave. For sense connection of "being one of the nation" and "free," compare Latin liber "free," from the same root as German Leute "nation, people" (see liberal (adj.)) and Slavic "free" words (Old Church Slavonic svobodi, Polish swobodny, Serbo-Croatian slobodan) which are cognates of the first element in English sibling "brother, sister" (in Old English used more generally: "relative, kinsman"). For the later sense development, compare ingenuity.

*dnghū-, Proto-Indo-European root meaning "tongue."

It forms all or part of: bilingual; language; languet; lingo; lingua franca; Linguaphone; linguiform; linguine; linguist; linguistics; multilingual; sublingual; tongue; trilingual.

It is the hypothetical source of/evidence for its existence is provided by: Latin lingua "tongue, speech, language" (from Old Latin dingua); Old Irish tenge, Welsh tafod, Lithuanian liežuvis, Old Church Slavonic jezyku "tongue;" Old English tunge "tongue; speech."