Originally Posted by Xfun

Kuai is a rude word but is used often and will get lots of laughs and giggles. Using the word in a personal conversation with the man you've just met is OK but will probably get a giggle.

But there's another reason why Thais will caution the use of the word Kuai - because it could easily be a mispronunciation of the Thai word for buffalo - kwai.

Or in a (hopefully) simpler explanation:

- Kuai - vulgar (but very often used) word for cock or penis
- Kwai - Thai buffalo (normal word for the actual animal)

Perhaps your intention is to talk about buffalos (the animal) in 'polite company' but there is the danger that you will say 'cock' rather than 'buffalo' and make everybody feel embarrased.

On the other hand, it's part of Thai culture (a nice part of Thai culture) to be careful of such things, but in reality (like in many cultures) such a 'mistake' would get lots of healthy laughs and giggles, rather than any anger or such.