Final answer:
Manga doujin originated in the mid-20th century in Japan as a popular practice among manga enthusiasts to create and publish self-made comics.
Step-by-step explanation:
The origin of manga doujin can be traced back to the mid-20th century in Japan. The term 'doujin' refers to self-published works, and 'manga doujin' specifically refers to self-published manga. The practice of creating and publishing doujinshi (fan-made comics) became popular among manga enthusiasts as a way to express their own stories and ideas inspired by existing manga series. These self-published works allowed fans to explore different themes, extend the storylines of their favorite characters, or even create original stories.