When my mother-in-law would get upset, she would say "Grub!". I continued this tradition and added it to Club Penguin. This is one of the original messages penguins can use to express their feelings of frustration. Others included the angry and skull emotes.
Grub appears in most of my games. Mech Mice makes reference to a food and weapon company called Grub Co and the mice say "Grub" when they are frustrated.