Thanks for the reference to the Wikipedia article. Very interesting! It also had a quote from a 1913 article which included a pre-musical definition of "jazz" and a confirmation that it was a new word at that time.

After reading that article and many others on the same subject, I think I see a kind of consensus on the history of the word "jazz":
- the word first appeared in print in 1912 in California, meaning "energy, spirit, spunk," in connection with baseball;
- it appears to have been a new word at that time;
- the word was used in print in that sense many times before it was ever used in print to describe music;
- the expression "jazz it up" is based on that sense, and was used commonly before the word was used in print to apply to music (which to me suggests that the expression might have led to the word being applied to that particular musical style, and it might also explain the 1915 writer's association of "jazz" with "blues");
- there was probably a concurrent and possibly earlier underground meaning that referred in some way to sex (as is the case with the word "spunk");
- it may have evolved from the much older words "jasm" and "jism," both of which probably had approximately the same set of meanings that "jazz" had before it was applied to music;
- it's first found in print in connection with music in Chicago in the 1915 article cited in the earlier post.