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]
How Did April 1 Become “April Fools’ Day”?How did this odd, prank-centric holiday come to be celebrated in the first place, and why is it called April Fools' Day?
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.