It is a sort of yo momma joke used in 'Diary of a wimpy kid' by
Greg Heffley
Substitute teacher: Greg, can you answer this problem?
Greg: Your Mama!
Subsitute tecaher: Excuse Me?
Greg: Your Big Fanny Granny!
Subsititute Teacher: Well I hardly think that's -
Greg: Your Slaphappy Grandpappy!