Definition of don't I know it! in US English:

don't I know it!

phrase

informal
  • Used as an expression of rueful assent or agreement.

    • ‘‘He's not the most forgiving person.’ Ryan laughed bitterly. ‘God, don't I know it.’’
    • ‘‘You know how kids tease other kids with unusual names.’ ‘Oh, don't I know it! Try having to grow up with the name Jasper!’’