This chapter sheds light on the semantic journey of chai ใจ, a key cultural concept in Thai culture. Chai is a person’s innermost secret part, linked to the body, with a dynamic capacity to move around and change its shape, size, colour, and even temperature, depending on the circumstances. The chapter surveys the lexical meaning and multiple ways in which chai is used in contemporary Thai, then takes a historical turn to examine the semantic development of the word from its earliest attested uses to the present day, relying on data from a wide range of classical and contemporary sources. The analysis reveals that the primary conceptuality of the personhood construct embedded in the word chai ใจ has remained relatively stable since its first appearance in the 13th century, when it was used to refer to a person’s disposition, representing a ‘locus’ where psychological activity occurs. The literary data in the later periods demonstrates massive growth in cognitive and cultural salience with its occurrence in a steadily increasing number of metaphorical expressions and idioms. The chapter concludes with the explication of the modern folk concept of chai ใจ, using English and Thai versions of the Natural Semantic Metalanguage (NSM) to avoid cultural bias.