Idioms and Phrases with nobody's fool
A person who cannot be duped or taken advantage of, as in You can't put anything over on Ryan—he's nobody's fool. [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.