This in turn is a reference to the phrase "putting on gomusin backwards" (Korean: 고무신을 거꾸로 신었다), which is a euphemism for being dumped while in the military.
[1] It is common to find gomsin among Korean women who are between twenty and thirty years old.
Gomusin first appeared in the early 20th century; they were easier to keep clean than danghye and jipsin (straw shoes) and they could be worn when during wet weather, attracting popularity and replacing traditional footwear.
[6] The term "putting gomusin on backwards" when applied to a couple, refers to the termination of their relationship because the girlfriend chose to pursue other men during the boyfriend's military service.
[1] In modern Korean society, there are many communies for gomsin, where users may commiserate with each other and share tips for enduring the long waits.