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.
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Source: A Dictionary of American Idioms