Posted on July 4, 2009July 30, 2018 by Idioms Poster Don’t chew with your mouth full You need some table manners, young man. Don’t chew with your mouth full! A mix up of: Don’t chew with your mouth open, and Don’t speak with your mouth full. From Anon in East Sussex, UK. Post navigationPrevious post: They treated me like a leopardNext post: It’s not rocket surgery