Не знаю, я читал книгу Thai Culture and Society. Там дается та трактовка, которая приведена у первого автора. В Википедию не лазил

Но слажу
Scruffy Westerners, especially backpackers, may also be called Farang khi nok. This means "bird-shit farang", as khi means waste and nok means (wild) bird; but, while khi nok may mean guano, it is also a species of fish, Diagramma pictum, a species of grunts Haemulidae.[3] Farang khi nok stating that Westerners are bird shit, is referring to the colour of a Westerner's skin, white, and the colour of bird shit, white. It also is a joke about farangs who arrive in Thailand by air since they are shit out of the big metal birds.
Вот, какие сложные ассоциативные ряды оказывается строит могучий тайский народ

Поневоле вспомнишь слово "черножопый"
