Clabbernong was the name of a family that lived in and around New Zebedee, Michigan and Capharnaum County at the turn of the 20th Century. Members of the family "had been farmers for generations, and they used to own hundreds of acres between New Zebedee and Homer” (The Beast under the Wizard's Bridge, 10).
While Brad Strickland created - or perhaps envisioned or interpreted - the Clabbernong family history, Bellairs first mentioned the characters in passing in The House with a Clock in its Walls when Lewis, his uncle, and Mrs. Zimmermann are chased across the iron bridge (House, 100).
Clabber is a food produced by allowing unpasteurized milk to turn sour. One wonders if Bellairs was envisioning a silly-sounding yet foul-tasting beverage called clabber-nog (i.e. similar to eggnog) when constructing the surname. If not, nong appears to be defined as "idiot" in some cultures.