It translates as "a person mixed up with morning urine" and is used to refer to a "simpleton" or "moron" (바보, 천치, 백치, 팔푼이, etc.).
I had never seen nor heard this expression until I came across it today among a list of proverbs. I am posting it because it seems it may have been based on an old Korean superstition. Did Koreans use to believe that mentally handicapped children were the result of their being born before the mother had a chance to take her morning pee?