only too well

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. © 2022 Farlex, Inc, all rights reserved.

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.’
See also: well
Farlex Partner Idioms Dictionary © Farlex 2017
See also:
References in classic literature ?
She had known Alfred Inglethorp only too well. I wondered whether, if she had remained at Styles, the tragedy would have taken place, or would the man have feared her watchful eyes?