The subject of this article is the idiom БОГЪ ВѢСТЪ / ВЕДАЕТ/ ЗНАЕТ, that is, БОГЪ ВѢСТЪ / Bog zna in Russian and Serbian. In the first chapter, which deals with the origin and genesis of the expression, the evidence of its biblical origin is given, while the process of its development as a phrase, its course and the reasons for the emergence of a new meaning of the idiom in communicative use are explained. The second chapter provides an analysis of the idiom from a paradigmatic aspect. Its lexical, morphological and stylistic variants are analyzed. The third chapter deals with the status of the idiom черт/бес/леший ЗНАЕТ in the Russian language, i.e., джବ¬vo zna (will znati, bi znao) in Serbian and its relationship to the idiom Бог весть/ ведает/ знает, that is, God knows. Descriptive, phraseological - monolingual and translation dictionaries - both contemporary and historical - were used as the basic corpus in the research. As an additional corpus, we used our own file created on the basis of excerpts of examples from literary and artistic works and epic poetry. The National Corpus of the Russian Language (NKRJ) served us as a basic check of the functioning of idioms in context. The application of the diachronic, contrastive and descriptive method on the aforementioned corpus was aimed at discovering the origin and genesis of the idiom highlighted in the title. The semantics of the expression Бог весть/ знает is identical to the semantics of the idiom черт знает, with the same syntactic structure, which unequivocally proves that it is an idiom of a structural-semantic model, but, on the other hand, despite the fact that both idioms have an identical global meaning, in the key component of the expression Бог i черт (God and the Devil) are made distinct, and are actualized today in the process of communication despite the phrasing of the meaning of the original expression. It is precisely this transparent internal form that causes a double actualization of the idiom in use and which means that the mentioned idioms in a specific context usually differ in their connotative value. Ta The different connotations of the idioms Бог знает and черт знает were created as a set of expressive (metaphorical and evaluative) elements related to the key components of the idiom, i.e. God or the Devil, The analysis of numerous examples of the contextual use of the two idioms in our literature confirms that apart from the meaning of uncertainty or doubt, which they have in common, they clearly also differ from each other in the connotative sphere. The idiom Бог знает (God knows) is used predominantly in situations that are accompanied by, if not positive, then at least neutral pragmatic content, while черт знает (the Devil knows) give rise to uncertainties and ambiguities, but often also expresses anger, displeasure, resignation or even rage. This type of differentiated relationship between the idiom and the component theonym and demonym can also be applied to the similar idioms used in Serbian.