- As if he'd done nothing at all.
- Which happens to be the truth.

5 seconds sound clip from the The Philadelphia Story movie soundboard.

You can hear this line at 00:53:38 in the Blu-ray version of the movie.

Quote context

[...]

- It'd look better coming from me, don't you think, as at least the titular head of the family?

- Of course, inasmuch as you let us in for it in the first place.

- Do keep that note out of your voice, Tracy. It's very unattractive.

- Oh? How does your dancer friend talk? Or does she purr? Sweet and low, I suppose. Dulcet. Very ladylike.

- You've got a heck of a nerve to come back here in your best head-of-the-family manner, and make stands and strike attitudes and criticize my fiancé, and give orders and mess things up generally.

- Stop it instantly!

- I can't help it. It's sickening.

- As if he'd done nothing at all.

- Which happens to be the truth.

- Anyway, it's not your affair, Tracy. If it concerns anyone...

- Well, actually, I don't know whom it concerns except your father.

- That's very wise of you, Margaret.

- What most wives fail to realize is that their husbands' philandering has nothing whatever to do with them.

[...]