Vulgar Words?

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In Central Thai shirt and believe have the same tone and typically falling tone words become low tone words in Essan and you're right that ช becomes ซ, which would make the words the same. However, in this case เชื่อ becomes เซือ (mid tone). เสื้อ does follow the normal trend and becomes เสื่อ. So to believe is mid-tone and shirt is low tone in Lao/Essan.

That is fascinating. I don't know very much about Lao/Essan, just words I pick up - are you saying that it has no (or few) falling-tone syllables?

It is interesting isn't it? They do still have lots of falling tone syllables. In general high-tone syllables in central thai become falling-tone syllables in lao/essan.