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.

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
See also:
  • only too
  • only too (something)
  • only too glad, ready, etc.
  • only too pleased
  • be glad to see the back of (someone or something)
  • be glad to see the back of someone or something
  • be glad, etc. to see the back of somebody/something
  • (boy,) am I glad to see you!
  • Am I glad to see you!
  • so
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?
The Mysterious Affair at Styles