Phineas: How'd you do that?
Reginald: It's just a little bit of "how's your father."
I may seem like a barmy ol' git now, but when I wasn't so long in the tooth, I had some grand larks and engaged in a fair amount of derring-do.
Phineas: Translate?
Ferb: When he was younger, he did a bunch of stuff.
Yes, he did a bunch of stuff, if you know what I mean
Sex. He did sex. "How's your father" is an old term for sex.