As long as you don't stand up in front of Father and do a back flip-flop.
2 seconds sound clip from the Holiday (1938) movie soundboard.
You can hear this line at 00:26:31.105 in the Blu-ray version of the movie.
Quote context
[...]
- Johnny, she says to me. She calls me Johnny.
- No, I'm having a swell time.
- No, but this is terrible, sister.
- Do you realize that you're trying to marry into one of America's 60 families?
- As long as you don't stand up in front of Father and do a back flip-flop.
- Can you do a back flip-flop? Can you really?
- Sure.
- You'll have to teach me. I can do almost anything else.
- It'll be a pity if this doesn't come off. It'll be a real pity.
- Yes, it'll be a pity if this doesn't come off too.
[...]