I lose count of the number of times I have been chatting with friends and someone will confidently explain where the word farang was derived.
One common explanation of the word’s derivation is that it is a simple contraction of the western word 'foreigner'.
Another explanation is that we are named after the guava fruit, rather than the other way around. Guavas are actually native to Central America so it is not quite clear how this could have happened before we had brought the fruit here.
Another commonly given explanation is that the first westerners the Thais met were the French and that 'farang' is a contraction of the Thai word for France - fa-rang-set. The big hole in this explanation is that the first westerners in Thailand were actually the Portuguese.
In fact, nobody is exactly sure of the word’s derivation but probably none of the above is right. Linguists generally believe the word is derived from the Arabic word 'frank', meaning foreigner. This does make sense as the way the Thai alphabet works would actually encourage Thais to add an inherent 'a' after the 'f' and to change the 'nk' at the end to 'ng', giving 'fa-rang'.