Thai, which is sometimes referred to as Siamese, is part of the Tai language family. The languages in this family belong to the much larger Austric language group. The spoken language is believed to have originated in the area which is now the border between Vietnam and China, an idea that provides clues to the origin of the Thai people, an area of continued academic debate.
The written Thai Language was introduced by the third Sukothai period king, Ramkamhaeng, in 1283. This writing system has undergone little change since its introduction, so inscriptions from the Sukothai era can be read by modern Thai readers. The writing was based on Pali, Sanskrit, and Indian concepts, and many Mon and Khmer words entered the language.
The earliest known example of the Indic-based writing system is the inscription of Ramkhamhaeng in northern Thailand from AD 1293. The Ram Khamhaeng Inscription, formally known as Sukhothai Inscription No. 1, is a stone stele bearing inscriptions which have traditionally been regarded as the earliest example of the Thai script. Discovered in 1833 by King Mongkut (Rama IV), it was eventually deciphered and dated to 1292. The text gives, among other things, a description of the Sukhothai Kingdom during the time of King Ram Khamhaeng, to whom it is usually attributed. The inscription had immense influence over the development of Thai historiography from the early 20th century, which came to regard Sukhothai as the first Thai kingdom.
The inscription is widely regarded as the single most important document in Thai history, and was inscribed on the UNESCO’s Memory of the World Register in 2003.
According to britannica.com, todaytranslations.com and en.wikipedia.org