It actually stems from the Latin word excellere meaning "to rise, surpass, be eminent." Broken down: ex- meaning "out from" and -cellere meaning "rise high." It's the same root from which we get the English words, excel and excellent. The word excelsior itself was the adjective form of excellere and means, simply, "higher" or "superior."
The State of New York adopted the word for their official seal (and flag) in 1778. The definitions at that time were generally given as "ever upward" or "ever higher" -- as someone apparently mistook the word as an adverb. The seal has remained essentially unchanged to this day.
This seal was allegedly seen by Henry Wadsworth Longfellow in 1841, and was the inspiration for his poem Excelsior...
In happy homes he saw the lightIt was this repeated use of the refrain "Excelsior!" that caught people's attention. In 1855, Walt Whitman penned a poem of the same name, as did Alexandru Macedonski in 1895. The Excelsior Brigade was a military unit in the Union Army during the American Civil War. In 1861, Sam Loyd applied the name to a unique chess problem he devised. In 1893, the name was given to the then-world's-largest diamond. Two car manufacturers took the name around the turn of the century and several motorcycle companies used the name in the early 20th century. P. G. Wodehouse penned a short story called "Excelsior" in 1948. In 1959-60, USAF Capt. Joseph Kittinger was involved in a "Project Excelsior" that was testing new parachute equipment and, in the process, set several still-standing world records. Not to mention several cities and townships that have been named "Excelsior" over the years.
Of household fires gleam warm and bright;
Above, the spectral glaciers shone,
And from his lips escaped a groan,
"Try not the Pass!" the old man said:
"Dark lowers the tempest overhead,
The roaring torrent is deep and wide!
And loud that clarion voice replied,
"Oh stay," the maiden said, "and rest
Thy weary head upon this breast!"
A tear stood in his bright blue eye,
But still he answered, with a sigh,
"Beware the pine-tree's withered branch!
Beware the awful avalanche!"
This was the peasant's last Good-night,
A voice replied, far up the height,
At break of day, as heavenward
The pious monks of Saint Bernard
Uttered the oft-repeated prayer,
A voice cried through the startled air,
A traveller, by the faithful hound,
Half-buried in the snow was found,
Still grasping in his hand of ice
That banner with the strange device,
There in the twilight cold and gray,
Lifeless, but beautiful, he lay,
And from the sky, serene and far,
A voice fell, like a falling star,
Among those who read Longfellow's poem was, not surprisingly, Stan Lee. Although I can't seem to find the reference now, I recall Stan saying that he appropriated the catch phrase from Longfellow.
Interestingly, it was a surprisingly late addition to the Lee-style hyperbole that ran rampant through those old Marvel comics. While there are plenty of "Face Front"s and "Nuff Said"s peppered through the earliest comics, the first instance of an "Excelsior" I can find is from a "Stan's Soapbox" in Fantastic Four #71 circa 1968. Of course, once he DID start using it, he never stopped! And, when questioned about it's meaning, naturally added his own unique spin to the definition by adding the "to greater glory" that's partially implied in Longfellow's poem.