완(完) means "complete, whole" 곡(曲) means "song" 완곡 means "a complete song" 완곡하다 means "to sing a complete song (at a time)" 앞으로 완곡하지마 means "Don't sing a complete song (at a time) in the next time" It means "You are a terrible singer."

완곡 has two different meanings Firstly, play some music completely. Secondly, indirectness I think 앞으로 완곡하지마 means "don't say indirectly ever"