Lay a finger on somebody
Definition 1 of 2
Pro Tip 1/3Conflict Avoidance
Used when promising not to start or engage in physical conflict. 
I said I wouldn't lay a finger on him, and I meant it.
Pro Tip 2/3Immediate Threat
Can imply the immediate possibility of physical harm. 
If you lay a finger on me, I'll call the police.
Pro Tip 3/3Protective Warning
Often used by someone warning others to not harm a person or animal. 
Lay a finger on my brother and you'll have me to answer to!