only too

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

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.
See also:
  • only too (something)
  • only too glad, ready, etc.
  • only too pleased
  • only too well
  • 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