Wiktionary
lay a finger on
vb. 1 (context idiomatic English) To merely touch 2 ''If you lay a finger on my little brother, I'll have your guts for garters''
Usage examples of "lay a finger on".
If you lay a finger on that boy, to hurt him, I'll lick this ugly cub of yours within an inch of his life!
I'll not leave a handful of that dark hair upon your head, if you lay a finger on me!
Yet I found I couldn't lay a finger on her other than to help her scrub the soap out of her hair.
When I quit seeing stars he was gone-and I hadn't managed to lay a finger on him.