What is georges reaction when slim calls Lennie a cuckoo?

He defends Lennie. He doesn't like it when people make fun of Lennie because of how George treated Lennie in the past, and he realised you shouldn't make fun of people or pull jokes on them because it hurts them, like when George told Lennie to jump in the river and Lennie couldn't swim. From then on, he stopped being mean and decided to look after Lennie.