By the end of the 1800s, “gism” meant not only “vitality” but also “virility,” leading to the word being used as slang for “semen.”
Note the discussion of what “jasm,” means, which suggests that it was fairly new, not in widespread use at the time. 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.”