Abstract: The object of our research is the problem of the integration of Spanish, Persian, Armenian and Slavic loanwords into the Greek language, while the subject is the etymology of μοῦτζος / μοῦτσος and μουζακίτζης. Special attention is paid to the problems of etymology and the nuances of dictionary entries throughout various time periods and source languages. It can be stated that the loanwords originated from various spheres of human activity, such as: seafaring, commerce, warfare, politics and government. We take in account both linguistic peculiarities and the usage context of μουζακίτζης, which is a hapax legomenon in Leo Deaconus’s Historia. The main conclusion of the study is that the most plausible version of the origin of μουζακίτζης is the Persian one (from موزک mūzak). We also provide solid counter-evidence against the Irano-Armenian and Slavic versions. The Spanish etymology of μοῦτζος / μοῦτσος (← mozo) is confirmed, possibly via Italian mozzo. The novelty of the study consists summarizing of the lexicological and lexicographic descriptions of the analyzed words and in the systematization of Byzantine and Modern Greek anthroponymy containing the Μο(υ)ζακ- element, as well as in the proposed conjecture into the text of Leo Deaconus’s edition, which assumes capitalization (Μουζακίτζης instead of μουζακίτζης). The relevance of the study is determined by the analysis of the loanword borrowing into the lexical fund of the Greek language within the context of the multilingualism of the Byzantine Empire, including the use of vocabulary, which has so far received insufficient attention in linguistics.