What does 물똥싸움 literally mean?

 ANSWER: a watery feces fight

물 means "water," 똥 means "feces," and 싸움 means "fight," so 물똥싸움 literally means "a watery (물) feces (똥) fight (싸움)," but it translates in English as "a water fight," when people splash water on each other for fun. Here 물똥 refers to "water drops" (물방울).

When children are having a water fight, it is better to use the phrase 물똥싸움 instead of just 물싸움" because 물싸움 can also mean "a fight over water rights," which, for example, can mean the right to use a water supply to irrigate one's rice field. So, to avoid confusion, it is better to use 물똥싸움 to refer to "a fun water fight" and 물싸움 to refer to "an angry water fight."

From Dong-A's Prime Korean-English Dictionary