How did "day" become a swear word?

Japanese characters are used to curse southerners. The audio-visual "In" of "Day" means to insert.

"Japanese" and "fuck" are the same meaning. How did it come from? It's like this: "Fuck" is vulgar, so some scholars use the word "gold". In some areas of our country, the pronunciation of the word "Jin" is the same as that of "Japan", so some swearing words like "Japanese mother" have appeared in some literary works.