From Sino-Korean 親舊 (친구): 親 (친, *chin*) meaning "close/intimate" + 舊 (구, *gu*) meaning "old/longtime." Together they literally describe someone who is a long-standing intimate — an "old close one."

In Korean culture, 친구 carries a more precise social meaning than the English "friend." It specifically refers to people of the *same age* (same birth year). Calling someone significantly older or younger 친구 can feel presumptuous or rude. Age-appropriate terms matter enormously in Korea's hierarchical social structure: older friends are 선배 (seonbae) or addressed with honorific titles, while the bond between 친구 (true age-peers) is uniquely close and informal, allowing the use of 반말 (banmal), the casual speech level.