fly on the wall

fly on the wall idiom
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® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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