The paper discusses a little known manuscript dictionary dating from the beginning of the 18th c. that is stored in the manuscript division of the Jagiellonian Library (Ms.Slav. Qu. 28 Lexicon slavo-rutenicum). In actual fact we are dealing with a trilingual (East) Slavic-Latin-German dictionary, which is unfinished (barely reaching the end of letter "o"), and consists of 746 pages in quarto format. The author is thought to be French polyglot and orientalist, Mathurin Veyssière de La Croze. The primary objective of the paper is to discuss the typological-etymological and functional-stylistic typology of the East Slavic vocabulary included in the analysed lexicon. A preliminary overview of the Slavic lexis in the Slavic-Ruthenian Lexicon suggests that the dictionary of the past is worth publishing, so it would become available to a broader group studying the history of East Slavic languages.