This article demonstrates how the field of Jewish interlinguistics and a case study of Jewish Russian (JR) can contribute to the general understanding of ethnolects. JR is a cluster of post-Yiddish varieties of Russian used as a special in-group register by Ashkenazic Jews in Russia. Differences between varieties of JR may be explained in terms of differing degrees of copying from Yiddish. The case of JR allows the general conclusions that (i) the diffusion of ethnolectal features into mainstream use is facilitated not only by a dense social network but also by a relatively sufficient number of speakers with a variety of occupations; and (ii) in addition to matrix language turnover and lexical and prosodic features, an ethnolect may be characterized by new combinability rules under which stems and derivational suffixes belong to the target language (here Russian) but their combination patterns do not.