Wumping Willow
Dang! This must be a Britishism, or Jo wouldn't have used it! But where did she get it?
How about Latin? Um, Moechor, to commit adultery. No good. There go all the romance languages.
Maybe it's Anglo-Saxon roots. Let's check in Cook. Hmm... Monig, many. Wom, defilement. Nope.
Irish? Moch, early. Uan, lamb. Nope, the early lamb doesn't get the worm.
German? Mögen, like, may. Wumit, with which. Nicht.
Russian? Mowg, brain. Well, that could be, but nothing for the other syllable.
Hindi - Mughal, elite. Yeah, been there; my probable root of 'muggle.' But then what? No...
Oh, well. If all else fails, we fall back on dad's old dictionary for the official derivation.
Ah! Finally, a little light on the subject:
Mugwump, an independent in politics. < Algonquian mukquomp, a great man.
Omigod!
The Sacred HP Texts have been sullied with an AMERICANISM??
Why, we drum people out of the fanfic corps daily for using Americanisms! How can this be? The world is ended!