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slang To tell the truth about something. "Fess" here is a shortening of "confess." Come on, fess up—you took the money that was on the counter, didn't you? Can you believe that guy fessed up to all three robberies?
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
fess up (to something)
to confess to having done something. (Short form of confess.) I tried to get the boy to fess up to doing it, but he wouldn't do it. Come on, fess up.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
To admit to something; confess something: The treasurer fessed up when confronted with the evidence of fraud. The gangster fessed up to the crime.
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.