And finally, the crackiest theory on Rincewind's parents arrives: Granny Weatherwax and Ridcully!
Back when they were young, Granny Weatherwax and Ridcully had a romantic relationship, which Ridcully remembers fondly--despite forgetting Esme's name. But what he does remember about her is she could run really fast.
And we already know who else runs fast. Rincewind!
Furthermore, Rincewind told Ridcully that his mother ran away before he was born.
And Granny Weatherwax declares she would do the exact same thing if she were a mother.
Now this is getting into semantics, but it could be that Granny doesn't consider herself a mother because she didn't raise her child, not because she never had one.
And sure, Granny is possibly asexual and it's likely her and Ridcully never got to that point in their relationship (which is why this is the crackiest theory), but both of them are magic.
Maybe somehow their magic spawned Rincewind, perhaps in a potato patch somewhere, without having to actually have sex or pregnancy to make him. Perhaps neither of them ever even knew this occurred.
Yeah, not likely, of course, but then everything about Discworld is not likely, so who knows?