[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.