This study examines the functional-semantic characteristics of the kinship terms “brother-in-law” in English and “qaynogʻa/qayni” in Uzbek from a comparative linguistic perspective. The research analyzes the morphological structure, etymology, and semantic scope of the English compound noun brother-in-law, highlighting its analytic and endocentric structure and its historical development from legal terminology derived from the Latin affinis. Particular attention is given to the polysemous nature of the term in English, which encompasses several affinal kinship relations and relies heavily on contextual clarification. In contrast, the Uzbek language employs a lexically differentiated system of kinship terminology, where distinct terms such as qayniuka, qayniaka, and kuyov precisely denote specific marital relations. The study also explores functional-semantic microsystems within both languages, demonstrating how Uzbek kinship terminology reflects a more detailed hierarchical and cultural structure. The findings reveal that while English kinship terminology tends toward semantic generalization and contextual interpretation, Uzbek demonstrates a highly differentiated lexical system that encodes social roles, generational relations, and cultural norms.