This paper deals with foreign lexemes attested in business and legal acts created in the area of Bosnia and Hum in the period ranging from the late 12th to the late 15th century, with the aim of shedding light on their origin, the domain of use and degree of adaptation within the Old Serbian language. In addition to the younger lexical layer, the focus was also on the old loanwords from the Proto-Slavic language. The excerpted corpus proves a strong influence of the Greek language, which, in addition to the field of ecclesiastical terminology, also left its mark in the naming of occupations, money and measures, and to a lesser extent in the legal sphere and secular titles, while it also mediated in the adoption of certain names of the months of the year. In addition to the calendrical lexicon, Latin and Italian covered the field of commerce, law, public services and organization of cities and fortifications. To a certain extent, the sources also provided insight into the ecclesiastical terms of Latin origin, as well as the Oriental titles adopted from Turkish. The Oriental influence appears sporadically even in the domain of politics and trade, whereas the words from the German and Hungarian languages are found only in individual examples. The incomplete adaptation observed within several thematic groups testifies to the ongoing and still current processes of establishing different terminological subsystems. Representing different chronological layers in the structure of the lexicon of the Old Serbian language, the lexicon of foreign origin thus largely reflected the specific political, economic and cultural context and influences that shaped the history of these areas in the Middle Ages.