5301991f_and_The_Art_of_Breaking_Things__detailed_explanation
[signal] Nail biting [part of body] mouth / hands [possible meaning(s)] frustration, suppression [detailed explanation] 
Nail-biting is an inwardly-redirected aggression borne of fear, or some other suppression of behaviour. Later nail-biting becomes reinforced as a comforting habit, again typically prompted by frustration or fear. Stress in this context is an outcome. Stress doesn’t cause nail-biting; nail-biting is the outward demonstration of stress. The cause of the stress can be various things (stressors).
“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