Origin of jismOrigin unknown
A fusion of what were originally two separate terms: "gism" and "jazz". For semantic development, compare spunk.
The term "gism" is first attested in print in 1842, with the meaning "energy". What appears to be a variant of the term, "jasm", appears in the 1863 book Miss Gilbert's Career: An American Story, where it is presented as a new term, once again with the meaning of "energy". However, use of the word to mean "semen" begins to appear by 1899, and no printed use of the term (in the specific sense of energy) is seen again until 1935 (in a Southern Californian publication) where it again is in the form "jasm". The form "jasm" may be derived from similar terms in the Niger-Congo languages (such as Mandingo jasi "energy"), but this doesn't account for the earlier "gism", which was used in the exact same sense, nor does it shed any light on the abnormal shift of medial /ɪ/ to /æ/.
Meanwhile, the term "jazz" seems to have originated in Southern California at some point in either the late 19th century or the early 20th century, perhaps initially as simply an Anglicised form of the Irish word teas "(literal) heat; passion" to be used in colloquial speech (creole-esque discourse) by some of the Irish immigrants to the region at the time. Its first occurrence in print is in 1912, in the Los Angeles Times, where it was used to describe the vigour and charisma observed in baseball players. It had spread as far northward as San Francisco by the early part of the second decade of the 20th century, again being used to describe baseball players. It was still at this point recognised as a new term by the locals of San Francisco, as can be seen in a description given by the (then) San Francisco Bulletin editor Ernest J. Hopkins, who called it "...a futurist word which has just joined the language." It was in San Francisco that local musician Art Hickman and his cohorts used the term to refer to a type of ragtime-derived music they had formed that would later come be called "sweet jazz". The term travelled with Hickman throughout his concerts in San Francisco and New York, and (although "sweet jazz" was an industrial "flop") (former) fellow band members like jazz music pioneer Bert Kelly continued to use the term. By 1914 the word (under the form "jass") had spread as far eastward as Illinois, where it was initially met with much public outcry in Chicago. Finally, in 1917, a letter from New Orleans native Freddie Keppard (who had settled in Chicago by this point) to cornet player Joe Oliver, used the term in relation to music. Keppard passed on the letter to his protégé Louis Armstrong. It was from there that the term began to be used for what was initially called "hot jazz", (so as to distinguish from Hickman's original "sweet jazz") and later came to be called simply "Jazz."
It is likely that "jazz" and "gism" fused to "jism" due to both gism's alternative form "jasm", as well as (perhaps) the subconscious wish to keep the terms separate due to the the emerging music genre (so as to not have it be affiliated with vulgar slang.)