The present study aims at investigating the functions and the meanings of /jaŋ/ from the Sukhothai period to the present, as well as, the grammaticalization of the word. The data collected is divided into 5 periods: (1) Sukhothai, (2) Ayutthaya, Thonburi, and Ratanakosin from the reigns of King Rama I to King Rama III, (3) Ratanakosin from the reigns of King Rama IV to King Rama V, (4) Ratanakosin from the reigns of King Rama VI to King Rama VIII, and (5) Ratanakosin during the reign of the present King It is found that /jaŋ/ in each period has 4 functions: (1) the verb -- /jaŋ/ in the first period had 3 meanings: ‘to exist’, ‘to stay or to be in/at’, and ‘to annex’. In the second period, /jaŋ/ also had 3 meanings: ‘to exist’, ‘to stay or to be in/at’, and ‘causative usage’. In the third and fourth periods, /jaŋ/ had 2 meanings: ‘to exist’ and ‘causative usage’. And in the last period, /jaŋ/ has only 1 meaning which is of the ‘causative usage’ used in some context. This diachronic study indicates that the verb /jaŋ/ tends to have fewer meanings towards the present. (2) the pre-verbal auxiliary -- /jaŋ/ has only 1 meaning of the ‘continuance aspect’ in every period. (3) the preposition -- in the first period, /jaŋ/ had 1 meaning indicating the case of ‘goal’. In the second, the third, and the fourth periods, the word had 2 meanings including the case of ‘locative’ and ‘goal’. In the last period, /jaŋ/ must occur with the motion verbs, especially /paj/ ‘go’ and /maa/ ‘come’. Hence, the word has only 1 meaning indicating the case of ‘goal’. (4) the conjunction -- in the first period, /jaŋ/ had only 1 meaning of the ‘additive relation’. The new meaning of ‘adversative relation’ found in the concessive construction was added in the second period. In the last periods, the word that has the meaning of ‘adversative relation’ is found in the context of the comparative construction. The study also reveals that the conjunction /jaŋ/ tends to develop more grammatical meanings towards the present. Another major finding in the present study is that the historical development of /jaŋ/ has 4 pathways: (1) the transition from the verb meaning ‘to exist’ to the pre-verbal auxiliary (2) the transition from the verb meaning ‘to exist’ to the conjunction (3) the transition from the verb meaning ‘to exist’ to the causative verb, and (4) the transition from the verb meaning ‘to stay or to be in/at’ to the preposition. Notice that the verb meaning ‘to exist’ is the source of the first 3 pathways. On the other hand, the verb meaning ‘to stay or to be in/at’ is the source of the last pathway. However, these 2 sources share the core meaning, i.e. ‘persistent state’. It is also found that the historical development of /jaŋ/ involves 5 mechanisms including metaphorical extension, generalization, absorption of contextual meaning, loss of constituent structure/reanalysis, and decategorialization. In conclusion, the grammaticalization of /jaŋ/ found in the data collected coincides with the universal characteristics of grammaticalization in term of gradualness and unidirectionality.
