This ethnographic case study investigated the disparity in an adult learner's second language (L2) proficiency between basic interpersonal communication skills (BICS) and cognitive academic language proficiency (CALP). The learner, well-educated in his first language (L1) Italian, demonstrated higher CALP but lower BICS in L2 English. Data collection spanned 15 weeks and included classroom observations, in-class artifacts, and semi-formal interviews. Two main factors were identified as contributing to the disparity: (1) unbalanced language exposure favoring CALP over BICS and (2) the positive transfer of CALP from L1 outweighing that of BICS. The study found that teacher instruction and learner's language engagement and investment increased exposure to grammar and literacy, thereby enhancing CALP development and facilitating CALP transfer from L1 to L2. However, limited exposure to interpersonal communication practices both inside and outside the classroom led to his underdeveloped BICS. Consequently, the unbalanced proficiency further influenced his second language acquisition. The learner's low BICS level resulted in negative self-positioning, increasing fear of interacting with English speakers and reducing opportunities to acquire the target language in natural interactive environments. Pedagogical implications are provided for adult EFL teaching, emphasizing balanced instruction and interactive and multilingual approaches. The research suggests future directions for supporting bi-/multilingual adults in developing balanced language proficiency and a positive language identity effectively.