An unseen observer or listener, as in I wish I could be a fly on the wall when she tells him she's pregnant. [Mid-1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
How to use fly on the wall in a sentence
Since 1984, Newsweek has made a cottage industry out of its fly-on-the-wall post-election coverage of presidential campaigns.
He could play the fly-on-the-wall in Louis the Goon's life as long as he wanted.