A coup is when some people try to take over a government by force, like a bully trying to take your toy away. A gapsin coup specifically refers to a military coup that happened in South Korea a long time ago in 1961. The military leaders of South Korea at the time thought that the government wasn't doing a good job and wanted to take control, so they used their weapons and soldiers to take over. It was kind of like a very big game of cops and robbers. The coup lasted for a while and afterwards, the military leaders became the new leaders of the country. It's not a great thing to do, because it can hurt people and cause chaos, which is why it's important to solve problems in a peaceful way.