Who told you Farang is derogatory? It may not be the hi-so term, but it is not an insult, though it can be used as one, just like many common terms.
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The word "farang" is generally not considered a derogatory term in Thailand. It is a neutral word that is used to refer to foreigners, specifically those of European or American descent. However, the word can be used in a derogatory way, depending on the context. For example, if someone is using the word "farang" in a negative way, such as to insult or belittle a foreigner, then it could be considered offensive.
The word "farang" is thought to have originated from the Persian word "farangi", which means "Frank". The Franks were a Germanic tribe that ruled Western Europe in the Middle Ages. The word "farang" was first used in Thailand to refer to the French, who were the first Europeans to arrive in the country in the 17th century. Over time, the word "farang" came to be used to refer to all foreigners, regardless of their nationality.
Today, the word "farang" is a common word in Thailand. It is used in everyday conversation and in the media. In most cases, the word is used in a neutral way. However, it is important to be aware of the context in which the word is used, as it can sometimes be used in a derogatory way.