6 .
"Dill and Jem were simply going to peep in the window with the loose shutter to see if they could get a look at Boo Radley, and if I didn't want to go with them I could go straight home and keep my fat flopping mouth shut, that was all"
The boys expect their peek into the Radley house to be straightforward, because of the "loose" shutter
Scout does not believe that the boys' plan will go as "simply" as they expect, although she decides to keep her mouth "shut"
Scout is not offended when Jem and Dill tell her to keep her fat flopping mouth shut
The reader is meant to understand that Scout is quoting Jem and Dill, who tell her rudely to keep her "fat flopping mouth shut"