Comment by anigbrowl

"gaijin" (rude slang for foreigner)

I think this is a misconception spread by people who get mad about being laughed at by locals, including kids, and have insisted on being called gaikukojin (foreign-country person) instead over the last 20-30 years.

The reason I say so is that Japanese is full of abbreviated words like this; gaijins literal meaning is 'outside person' and the koku part is redundant. You see such abbreviations in the written language too. For example 友人 (yuujin) and 友達 (tomodachi) both mean 'friend' but as you can see the latter kanji is a lot more work to write as well to say.

The real reason (in my view) is that this word 外人 has the same pronunciation as 害人 and the same slang meaning, but 害 carries an implication of harm or injury. Japanese has a small number of sounds compared to other languages so homophony abounds and double meanings like this are very common, both for humorous effect or for making veiled negative comments.

Switching a character around is a normal Japanese way of dealing with meaning clashes. For example 和 (wa) means harmony, but also refers to Japan: 和食 (washoku) is Japanese food, 和服 (wafuku) is Japanese clothing etc. etc.. This word is ancient, going back ~1800 years to when Chinese & Korean explorers first had contact with Japan and called it the 'Kingdom of Wa'. However the Chinese used the character 倭 (also wa) which means distant, but can also mean dwarf (as in stature) or submissive. The Japanese used the same character for about 500 years but eventually decided they found the double meaning offensive and switched to 和. https://en.wikipedia.org/wiki/Wa_(name_of_Japan) is a great deep dive into this if you're interested in etymology.

I get why some people take umbrage at the use of words like gaijin, but to my mind if you don't like such things you don't really like the language, and if you make an issue out of it people will just find more subtle ways to express their negative (and possibly escalated) sentiments and might start to view you as 害虫.