The Thai greeting referred to as the wai (Thai: ไหว้, pronounced [wâːj]) consists of a slight bow, with the palms pressed together in a prayer-like fashion. It has its origin in the Indic Añjali Mudrā, like the Indian namasté and the Cambodian sampeah. The higher the hands are held in relation to the face and the lower the bow, the more respect or reverence the giver of the wai is showing. The wai is traditionally observed upon entering formally a house. After the visit is over, the visitor asks for permission to leave and repeats the salutation made upon entering. The wai is also common as a way to express gratitude or to apologise.
The word often spoken with the wai as a greeting or farewell is sawasdee (สวัสดี, RTGS: sawatdi, pronounced [sàwàtdiː]). This word was coined in the mid-1930s by Phraya Upakit Silapasan of Chulalongkorn University. This word, derived from the Sanskrit svasti (meaning "well-being"), had previously been used in Thai only as a formulaic opening to inscriptions. The strongly nationalist government of Plaek Pibulsonggram in the early 1940s promoted the use of the word sawasdee amongst the government bureaucracy as well as the wider populace as part of a wider set of cultural edicts to modernise Thailand.
Waiing remains to this day an extremely important part of social behavior among Thais, who are very sensitive to their self-perceived standing in society. As a rule of thumb for foreign tourists and other visitors unaccustomed to the intricacies of Thai language and culture, it is unwise to initiate a wai exchange with someone who is younger. However, one should always return a wai that is offered as a genuine sign of respect. Corporate wais, such as those performed by convenience store cashiers, can generally be 'returned' with a smile.
If one is waiied while carrying goods, or for any other reason that makes the physical act of returning a wai difficult, it is still important to recognize the show of respect and make a physical effort to wai back as best as possible under the circumstances.
The wai originated from an ancient greeting that was done to show neither individual had any weapons. There exist multiple versions of the greeting based on social class, gender, and age. The gesture may come from Buddhism, which sometimes involves prostration, or clasping the hands palms together and bowing to the ground. The gesture first appears c. 4000 years ago on the clay seals of the Indus Valley Civilization. 
Similar gestures in other countries
In Indonesia, wai like gestures are in use in various parts of the country, Java (called susunan or sembah in the royal courts), Lombok and Bali, where Hinduism and Buddhism is or has been widely practiced.
In Laos and Cambodia, similar greetings—called nop (ນົບ) and satu, respectively—are also in use.
In Malaysia and Brunei, it was historically used to convey thanks or salutations to a patron or higher personage, with the hands raised to a level in accordance with the rank or caste of the individual to whom it was directed. It is still used in the presence of Malaysian or Bruneian royalty.
It is commonly used to greet people in India
Although not used as a greeting gesture, similar gestures (the clasping of hands at the stomach, chest or chin) are known in the Philippines to convey heartfelt gratitude to a helper or benefactor, especially if that benefactor's social status is above that of the one who is assisted. This has its origins in the pre-Hispanic and pre-Islamic Hindu-Buddhist beliefs and customs of the area. It is still used as a salutation before and after the pangalay dance of the Tausug and Bajau peoples of the Sulu Archipelago.
- Phya Anuman Rajadhon, Thai traditional salutation, Thai culture Series no. 14, The Fine Arts Department, Bangkok, Thailand, 1963
- Barmé, Scot (1993). Luang Wichit Wathakan and the creation of a Thai identity. Institute of Southeast Asian Studies. p. 176. ISBN 978-981-3016-58-3.
- http://www.csuchico.edu/~cheinz/syllabi/asst001/fall97/2chd.htm Economics of the Indus Valley Civilization by Chad Greenwood]
- Diller, Anthony (1991). National Identity and Its Defenders. Chapter 4: "What Makes Thai s Central Language". ISBN 974-7047-20-9.
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