1. Language of the Franks or a Frankish language.
2. A common language consisting of Italian mixed with French, Spanish, Greek, Arabic, and Turkish; the mixed language used chiefly by merchants throughout the Mediterranean ports until the 18th century.
3. Any hybrid or other language used over a wide area as a common mode of discourse between groups or peoples speaking different languages; especially as a commercial or trade jargon.
4. A useful makeshift lingo or commercial tongue among peoples of different speech; such as, English and a local language in various countries.

The Frankish tongue.

A jargon used for elementary communication by medieval traders and sailors in the Mediterranean, derived for the most part from the Romance languages [Italian mixed with French, Greek, Arabic, etc.]. Generally, it describes a mixture of languages used as a means of communication in business.