A performance credited name created from combining Murray Kaufman aka Murray the K and his mother Jean.
Murray's mother Jean suggested that "Splish Splash I was Taking a Bath" would be a good first line for a song. Bobby Darin and Murray wrote the lyrics; Murray's mother wrote most of the music.
The song was credited to "Bobby Darin & Jean Murray" to avoid any possible suggestion of payola if the song became a hit
(Story told at SongFacts and retold in slightly different versions elsewhere.) Also See - Splish Splash @ Wikipedia