The name “Phra Somdej” refers to one of Thailand’s most famous Buddhist saints in history: Phra Buddhācariya Toh Brahmaraṃsi (Thai: พรหมรํสี) – or in short: Somdej Toh (Thai: สมเด็จโต) – (B.E. 2331-2415; C.E. 1788–1872). “Somdej” is, in fact, the title of the highest ranking a monk can be given by the King (Somdej Toh would receive his title from King Chulalongkorn).
Somdej Toh was possibly an illegitmate child of King Rama II, and throughout his life time he would keep close ties with the royal court. Somdej Toh belonged to the Mahānikāya sect within Thai Theravāda Buddhism, which made him the most important monk of the settled monastic community. During the time he spent in the monastry he managed to become a scholarly expert in studies of the Pāḷi Canon. In a relatively short period of time Somdej Toh would receive recognition (from his elders) for his scholarly degrees in the monastic studies of the Pāḷi Canon.
However, apart from being a monk living in a settled monastic community Somdej Toh also spent much of his time wandering in the forest as forest monk. When Somdej Toh had stayed in the forest for a longer period of time he would always make special amulets for the laypeople living in nearby villages in order to thank them for the food they offered to him when he passed through the village on his daily alms round. These amulets served as sacred objects for contemplation of the Three Jewels (the Buddha, the Dhamma and theSaṅgha).
The laity regard Phra Somdej amulets as a manifestation of Somdej Toh’s saintly charisma. In this way, a cult arose around Somdej Toh and his amulets. The original design of Phra Somdej amulets created by Somdej Toh became increasingly popular. Whereas Phra Somdej amulets at first were created only by the three (major) royal temples – Wat Gaes Chaiyo, Wat Bang Khun Prom, Wat Rakhang Kositaram – they soon would also be made in many other Thai temples.
This has remained so ever since; Phra Somdej amulets are seen as the most sacred and desirable amulets of Thailand.