5301991f_and_The_Art_of_Breaking_Things__detailed_explanation
[signal] Bottom lip jutting out [part of body] mouth [possible meaning(s)] upset [detailed explanation] 
Like rubbing eyes, this can be an adult version of crying, so jutting or pushing the bottom lip forward is a part of the crying face and impulse. Bear in mind that people cry for reasons of genuine upset, or to avert attack and seek sympathy or kind treatment.