''On April 5, 1913, the Bulletin published an article by Ernest J. Hopkins entitled "In Praise of 'Jazz,' a Futurist Word Which Has Just Joined the Language." The article, which used the spellings "jaz" and "jazz" interchangeably, discussed the term at length and included a positive definition: "Jazz" (We change the spelling each time so as not to offend either faction) can be defined, but it cannot be synonymized. If there were another word that exactly expressed the meaning of "jaz," "jazz" would never have been born. A new word, like a new muscle, only comes into being when it has long been needed...This remarkable and satisfactory-sounding word, however, means something like life, vigor, energy, effervescence of spirit, joy, pep, magnetism, verve, virility, ebulliency, courage, happiness – oh, what's the use? – Jazz.''

--Richard Sudhalterr, from Lost Chords: White Musicians and Their Contribution to Jazz

online etymology:

"The word “jazz” probably derives from the slang word “jasm,”which originally meant energy, vitality, spirit, pep. The Oxford English Dictionary, the most reliable and complete record of the English language, traces “jasm” back to at least 1860...   Some have suggested that it originated as a variant of “gism,” which has the same meaning and can be traced back a little further, to 1842. By the end of the 1800s, “gism” meant not only “vitality” but also “virility,” leading to the word being used as slang for “semen.”

cf. spunk having the double meaning of 'gumption, exuberance, nerve, get-up-and-go etc' and, well, spunk...