Onomastic: of or relating to the study of the history and origin of proper names. Which is not how Garner uses it- he means 'the invocation of proper names for some purpose'
Apotropaic: having the power to avert evil influences or bad luck.
Euhemerism: "the philosophy attributed to Euhemerus which holds that many mythological tales can be attributed to historical persons and events, the accounts of which have become altered and exaggerated over time."
There's also a vague but pungent scent of smrtrthnu to the man. "Why, I had no difficulty at all in understanding Preiddu Annwn; it's perfectly clear to *me*." Good for you, bunny; now put on your shoes, I'll walk you to the subway.