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.
“I pretty much try to stay in a constant state of confusion just because of the expression it leaves on my face.” – Johnny Depp | Zen and The Art of Breaking Things
detailed explanation