Chaiyo (ไชโย, pronounced [tɕʰāj.jōː]) is a Thai-language exclamation used to express joy or approval, comparable to 'hurrah/hooray' in English.
When his Wild Tiger Corps retinue failed to respond in unison to the traditional yodel-like call and response ho-hiw,[a] Vajiravudh instructed them to shout yo in response to the call chai, hence completing the cheer chai-yo, three times in succession (much like hip hip hooray in English).
[1][2] In later writings, Prince Damrong Rajanubhab noted that chaiyo as an interjection was actually an old term, found in Thai poetry from the reign of King Rama II (1809–1824), but which had fallen out of common usage by then.
[4] According to the Royal Institute Dictionary, chayo is a variant form of ชัย (chai), itself a loanword from Pali/Sanskrit jaya (जय), meaning 'victory'.
[6] The poetic use of chayo remains familiar as it is the final word in the royal anthem Sansoen Phra Barami.