Theodiscus, the Latinised form of a Germanic word meaning "vernacular" or "of the people", is a Medieval Latin adjective referring to the Germanic vernaculars of the Early Middle Ages. The Old High German language in Latin sources of the time is referred to as theodisca lingua. It also appears as theudiscus, theotiscus, thiudiscus. The various Latin forms are derived from West Germanic *þiudisk and its later descendants.

The use of theodisce/deutsch was first attested in 786 in a report to Pope Hadrian I. Texts from a synod held in Corbridge, England were read tam latine quam theodisce "both in Latin and in the vernacular".

It is derived from Common Germanic *þiudiskaz. The stem of this word, *þeudō, meant "people" in Common Germanic, and *-iskaz was an adjective-forming suffix, of which -ish is the Modern English form. The Old English form is þéodisc, the Old High German one diutisc (attested ca. 1090 in the Annolied). The contrasting term appears to have been walhisk ("Welsh"), which was used to refer to Celtic and Roman (Gallo-Roman), but not Slavic, populations.