What is the meaning of the word rattapallax?

It's an onomatopoeia for the sound of thunder. It appears in Wallace Stevens' poem "Frogs Eat Butterflies, Snakes Eat Frogs, Hogs Eat Snakes, Men Eat Hogs." One line reads "Heavy with thunder's rattapallax." It sounds like Stevens may have taken it from "rataplan" (from a French onomatopoeia), a word for a drumming or beating sound.

http://www.porkopolis.org/pig_poet/wallace-stevens/
https://en.wiktionary.org/wiki/rataplan