This paper documents fieldwork on the Ugong language, a Tibeto-Burman language of the Burmese-Lob subgroup, spoken by a small community in western central Thailand. Over three periods (1977, 1979–80, and 1988), the study traces the rapid decline of Ugong due to social and economic changes, increased intermarriage, and language shift to Lao and Thai. Intensive fieldwork in Kok Chiang village reveals both the linguistic and social dynamics of language attrition, including the development of a Ugong writing system and community efforts at maintenance. The research highlights processes of phonological and morphological attrition, dialect convergence and divergence, and adult dialect adaptation. Documenting Ugong’s terminal language shift offers valuable insights for linguistic theory and supports ongoing cultural and linguistic preservation(AI generated abstract, Copilot)