1
general::
Also, put a finger on. Barely touch, as in You'd better not lay a finger on those documents! or If you lay a finger on me, I'll sue. This expression is nearly always used as a prohibition. [Mid-1800s] Also see PUT ONE'S FINGER ON.
American Heritage Idioms
2
general::
Phrase(s): lay a finger on someone or something
to touch someone or something, even slightly, as with only a finger. • Don’t you dare lay a finger on my pencil. Go get your own! • If you lay a finger on me, I’ll scream.
McGrawhill's American Idioms And Phrasal Verbs