5301991f_and_The_Art_of_Breaking_Things__detailed_explanation
[signal] Chewing gum [part of body] mouth [possible meaning(s)] tension, suppression [detailed explanation] 
As above – an inwardly-directed ‘displacement’ sign, due to suppression of natural reaction. Otherwise however can simply be to freshen breath, or as a smoking replacement.
“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