What does 백발이 성성한 노인 mean?

ANSWER: a gray-haired old man

백발 (白髮) means "white (白) hair (髮)," and 노인 (老人)  means "old (老) man (人)," but what does 성성한 mean?

성성(星星)하다 is a Sino-Korean word that means "grizzled" or "gray-streaked," so 백발이 성성한 노인 would be more accurately translated as "an old man with gray-streaked hair," though many Koreans mistakenly think it means the old man's hair is completely gray.

성성(星星)하다 literally means "star (星) star (星)," which means it could be translated as "starry." It is the same 성 that is used in the Korean names for the planets. For example, the Korean name for  Mercury is 수성 (水星), which literally means "water (水) star (星)"; and the Korean name for Mars is 화성 (火星), which literally means "fire (火) star (星)."

So, that means that 백발이 성성한 머리 literally translates as "starry white hair," which suggests that it is as black as the night sky with starry specks of white in it, something that might be called "salt-and-pepper hair" in the United States.

Dong-A's Prime Korean-English Dictionary (1998)