lay a finger on

lay a finger on  {v. phr.}

To touch or bother, even a little. — Used in negative, interrogative, and conditional sentences.

Don't you dare lay a finger on the vase!

Suppose Billy fakes his brother with him; wilt the mean, tough boy down the street dare lay a finger on him?

If you so much as lay a finger on my boy, I'll call the police.

Compare: LAY HANDS ON, PUT ONE'S FINGER ON.