lingua franca

This is an Italian phrase dating back to 1553 and referencing the language of the Franks, that is, the people of Western Europe in the late Byzantine Empire.  There was, in the 1600s, a pidgin based on southern European Romance languages  mixed with Arabic and other Mediterranean languages.  It was used as a language of commerce and diplomacy in the Mediterranean and then in northern Africa and the Middle East. By the mid 1600s the phrase was established in English, originally with reference to a pidgin or mixed language of some kind, but then later describing any language that is shared and allows for general communication.

Just out of interest, the Franks were so called because they favoured the javelin. The Old English word for a javelin was franca.  Compare this with the Saxons who favoured the knife (Old English seax).  The idea that frank meant ‘free’ was around in the 10th century but was a bit of folk etymology.