lay a finger on
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.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.