“Soft Kitty” is a song that Sheldon’s mother used to sing to Sheldon when he was sick as a child. Today, he still requests his caretaker to sing this song to him whenever he feels ill. Sometimes he also sings to others who have fallen ill.
The song is originally called “Warm Kitty”, and parts of its lyrics sequence have been altered for the version sung in The Big Bang Theory.
The song is first introduced in “The Pancake Batter Anomaly” (S01E11) where Penny sings it to Sheldon when he has caught a cold.
The daughters of the original songwriter filed a lawsuit on the show in December 2015 for using the altered version of the song without permission. Because of that, the song has not been performed after Season 8. The lawsuit charges have since been dropped.