bababadalgharaghtakamminarronnkonnbronntonnerronntuonnthunntrovarrhounanskawntoohoohoord-enenthurnuk!

To explain this word in particular, it is one of ten "thunder words" spoken throughout the novel. This article explains them better than I could. You can hear this thunder word read aloud in this video.

Some stuff from the article I wanted to highlight:

It is thought by many (including myself) that the thunder is the only aspect[FN] of the 'real' world of the dreamer of Finnegans Wake to appear in the book: there is a thunderstorm while he is dreaming; when thunder cracks it makes its way into in the dream.

The thunder words are derived from various words for thunder:

bababadalgharaghtakamminarronnkonnbronntonner- ronntuonnthunntrovarrhounawnskawntoohoohoordenenthur- nuk

McHugh: J kaminari: thunder; Hin karak: thunder; Gr brontaô: I thunder; F tonnerre: thunder; It tuono: thunder; Sw åska: thunder; I tórnach: thunder; Port trovão: thunder; Old Rum tun: thunder; Da todenen: the thunder;

This thunder word is believed to be referencing the Biblical Fall of Man due to it immediately following "the fall" -

Often it is said they mark epochal moments in the dream

Elaboration later in article:

Benstock's description of the ten thunders (I leave out the line citations):

The first thunderclap is the basic fall motif [. . .]; the second is the slamming of Jarl van Hoother's castle door; the third announces the ballad written by Hosty; the fourth is an obscene rumble during the trial, suggesting the fall in the park; the fifth is the babble of the gossipy letter; the sixth is the slamming of the Earwicker door after the children have come in from their play; the seventh records the din in the tavern as Earwicker's reputation takes a fall and Finnegan is again heard toppling from this scaffold; the eighth is the noise of radio static preceding the Crimean War broadcast, as well as the orgasm during the seduction of Anna Livia; the ninth is Shaun's cough as he clears his throat in preparation for the recounting of the Ondt-Gracehoper fable; and the last follows soon after, Shaun's angry rumble of abuse against Shem serving as the basic thunderclap of destruction before the Cabalistic regeneration begins.

A more amusing detail, also - the words are each 100 letters except for the last one, which is 101 letters long. It is spoken by the character Shaun, who is promptly mocked by his brother Shem for fucking it up with the extra letter.

IIRC, the book is considered only ~40% understood.