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off-the-books

American  
[awf-thuh-books, of-] / ˈɔf ðəˈbʊks, ˈɒf- /

adjective

  1. not recorded in account books or not reported as taxable income.

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.