Looking at the components of "thank you" in different languages may tell us something about different cultures. In particular, we might learn how people express gratitude by analyzing how the phrase "thank you" in their language is formed.
While I do not quite understand "thank you" in Thai and Lao, I think I understand "thank you" in other Southwestern Tai languages better.
The word for "thank you" in Lao is ຂອບໃຈ (ขอบใจ) khɔ̏ɔp.cài. In Thai, it is ขอบใจ khɔ̀ɔp.cai (cf. Lao ຂອບໃຈ) and more politely ขอบคุณ khɔ̀ɔp.khun. The latter is probably not as traditional since it contains an Indic loan: คุณ guṇa; so, I will leave this one out. The literal meaning of ขอบใจ in both languages is "the brim of heart", which quite frankly is really puzzling to me. In particular, I do not know how this is being used as "thank you". Unless perhaps, it does not mean "brim of heart" at all. Or maybe it is too poetic beyond my comprehension. Or maybe when a speaker says "the brim of heart", he is trying to say that his heart is full to the brim. Or perhaps he is trying to show the size of his heart by mentioning the brim? Oh boy, I am over-analyzing this. (If you know the answer, please let me know.)
On other hand, the literal meaning of "thank you" in some other Tai languages is not as puzzling (to me at least); it is quite poetic actually. In Northern Thai and Tai Lue, "thank you" is ยินดี nyin.dii or yin.dii, which means "welcome" in Thai and Lao. "Thank you" in these two languages literally means "(I) feel good", composing of ยิน "to feel" and ดี "good". In Shan, it is ငိၼ်းၸူမ်း ngín.cóm, which cognates with Thai ยินชม yin.chom. The literal meaning of Shan "thank you" is "(I) feel pleased, glad in the heart", composing of ngín "to feel" and cóm "to be pleased, to be glad in the heart". Thus, in these languages, to express gratitude, one says "I feel good, or I feel pleased".
So, the next time someone does something nice for you, do not forget to say "I feel good!" and give them a big smile. :D
Anyway, to me "I feel good/pleased" makes a lot more sense than "brim of heart". What do you all think?