A God who apparently started life on the Disc by sacrificing his unmentionables and casting them at his father, the Sun-God. Noddi is only believed in by the people of NoThingfjord and is not one of the big players at Dunmanifestin. The subject of a theological discussion among the Wizards as to how best to spark intelligent life on Roundworld, the general consensus was that it called upon too great a sacrifice for any thinking Wizard to contemplate. The Lecturer in Recent Runes voiced the more speakable of two possible options when he implied that he wasn't taking his underwear off for any cause, however good. Other Wizards allowed him to maintain his illusion. Noddi's indeterminate sacrifice is debated in The Science of Discworld.
This may well be a nod (sorry) to Noddy, a children's character beloved in the UK but conventionally subject to ridicule from adults to such an extent that the very name "Noddy" is a joke.
Also see Noddy Holder, another British institution.