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.

only too

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.

only too ˈglad, ˈready, etc. (to do something)

very glad, ready, etc: If you want any advice, I’d be only too willing to give it. ♢ She was only too pleased to help.

only too ˈpleased (to do something)

very happy or willing to do something: We’re only too pleased to help.

only too ˈwell

used for emphasizing that you already know about somebody/something or have already experienced something unpleasant, etc: ‘Do you know Alex Humber?’ ‘Only too well. He’s not one of my favourite people, I’m afraid.’