I think “dad” is short for “daddio”, but I’ve never heard it used like that. Bonus panel:

Was originally looking for this panel, but decided it was too straightforwardly domestic abuse and/or fetishy. Thought the posted panel was better anyways, though. Transcript of this panel:

Chuck: I’m going to spank you, Mrs. Brant… just a reminder that I’m your husband!

Diane: Don’t you dare, you… Ohh, not so hard!

Edgar: Er… care for another dance, Mrs. Brant?

Diane: N-no… I think I’ll sit… I mean stand around a while!

I also came across an interesting article:

Let’s put a bit of context on this. Love stories aimed at a predominantly female readership have been around since at least the early twentieth century, and through much of that time some of them have included spanking scenes. In the comic books, romance really got started as a distinct genre after the Second World War, and there was a very noticeable spike in the number of spanking scenes in 1949-50: a staggering seventeen are known from those two years, ten of them concentrated in the second half of 1949 alone.

“Please, Diane – Dad is my father’s name.”