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only too (something)
Very. Don't tell her the truth—she'd be only too glad to use it against you later. As a small business owner, I know only too well how tough it is to succeed in this economy.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
1. At the very least, as a matter of fact, as in I know only too well that I can't win the lottery. This usage was first recorded in 1817.
2. Very, extremely, as in I am only too glad to help. This usage was first recorded in 1899.
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.