This is the first word that we will use to greet each other. It might also be the first Thai word that you know but in reality it is not Thai. Actually, this is a Sanskrit word, created by Phraya Upakitsinlapasan (1879 – 1941) for greeting between students and professors in the faculty of Liberal Arts, Chulalongkorn University when he worked as a professor there. It became the official greeting word in 1943 by Field Marshal Plaek Phibunsongkhram (1897 – 1964), the Prime Minister of Thailand at that time.
สวัสดี /sà-wàd-dēe/ is a compound word from a prefix สุ /sù/ (meaning good, beautiful, easy) and a word อสฺติ /àd-sà-dtì/ (meaning there is, there are, have). When these two words merged together following the grammar rules, สุ /sù/ became สว /sà-wà/. Plusing อสฺติ /àd-sà-dtì/, สวสฺติ /sà-wàd-dì/ means ‘Wish (you) goodness and virtue!’ It reminds me of the phrase “May the force be with you!” Does it ring a bell? LOL
This word was originally a short vowel sound for all syllables but Phraya Upakitsinlapasan changed the last syllable into a long sound to make it sound better.
After that, there were some more greeting words created such as อรุณสวัสดิ์ /à-run-sà-wàd/ for ‘good morning’, ทิวาสวัสดิ์ /tí-waa-sà-wàd/ for ‘good afternoon’, สายัณห์สวัสดิ์ /săa-yan-sà-wàd/ for ‘good evening’ and ราตรีสวัสดิ์ /raa-dtee-sà-wàd/ for ‘good night’. However, these words are not as popular as สวัสดี /sà-wàd-dēe/ which we can use any time during the day. Only อรุณสวัสดิ์ /à-run-sà-wàd/ for ‘good morning’ and ราตรีสวัสดิ์ /raa-dtee-sà-wàd/ for ‘good night’ are still used until now.
May the virtue be with you!