"Neck No Nothing" or "Neck Know Nothing" is a slang term used when something embarrassing is said or done. It is also used if something obvious has just been stated and then somebody asks about it. The term is also connected to " Gimme That Neck ".
Teacher: "Today we will be learning about Poetry."
Student 1: "What are we learning about?"
Student 2: "Poetry! The teacher just said that!"
Student 3: "Neck!"
Student 4: "Neck No Nothing!"
Student 3: "Gimme that neck!"