What does 고인(故人) mean?

ANSWER: a dead person

The Chinese character 故(고) means "ancient," and 人(인) means "person," so 고인(故人) literally means "an ancient person." Since ancient people are dead people, if you become an ancient person, you become a dead person. That is why the phrase 고인 되다 means "to die."

But 고인(故人) can also mean "an old friend" since 고(故) is the same 고 used in 고향(故鄕), which means "hometown" or one's "old (故) village (鄕)." So, 고인(故人) or 고우(故友) is another way to say 고향 친구, which means "hometown friend" or "childhood friend."

By the way, the Korean word for "dolmen" is 고인돌. Even though it is supposedly a pure Korean word, I wonder if it originally meant "dead people rocks" (故人돌).