Yours, Etc.: Origins and Uses of 8 Sign-Offs
Latin excelsior "higher," comp. of excelsus (adj.) "high, elevated, lofty," past participle of excellere (see excel), taken 1778 as motto of New York State, where it apparently was mistaken for an adverb. Popularized 1841 as title of a poem by Longfellow. As a trade name for "thin shavings of soft wood used for stuffing cushions, etc.," first recorded 1868, American English.