Monumentous: Very significant. A portmanteau of "monumental" and "momentous."
Reflecting on Barack Obama's win in last week's Iowa caucuses, New York Times columnist Bob Herbert wrote:
Mr. Obama has shown, in one appearance after another, a capacity to make people feel good about their country again. His supporters want desperately to turn the page on the bitter politics and serial disasters of the past 20 years. That they have gravitated to a black candidate to carry out this task is — to use a term I heard for the first time this week — monumentous.
Monumentous as a descriptor for "the Obama phenomenon" has been in circulation for more than a year. A commenter on the From the Left blog wrote in December 2006: "The next election may be the most monumentous since the elections at the end of the Civil War and end of the Great Depression." And in December 2007 a commenter on an MSNBC blog wrote, "The thought of Obama winning the democratic nomination is absolutely inspirational and monumentous."
I heard a lawyer today refer to a “monumentous mistake”—a wonderful portmanteau word, if not precisely an eggcorn. Googling turns up 35,400 hits, which ain’t bad. Here’s a quote I like, from a piece of would-be poetical prose:
when it happens to an elephant, you think that is tremendous, you think that’s it, kneel down and rub your forehead, that is a monumentous sad death, something monumentous has passed from the earth, or in zoos when an elephant sways you think, something monumentous has gone psychotic, or something monumentous has broken through its enclosure and is stamping the crowds to death in the circus arena . . .
The earliest citation may this one, from February 2004: "Monumentous task of making a list of all DDoS Zombies."
UPDATE, Jan. 2009: Turns out "monumentous" began appearing in print much earlier than that--as early as 1896, according to a Google book search. In most instances, it was set off in quotation marks or otherwise marked as an invented or unusual word. But I still haven't found it in a dictionary. Anyone?