In response to Ed Schulz's query on the origins of "jizz," David McDonald
in Australia did some pretty thorough research on the subject about a year
ago, and published the results, both in a journal and in email form. You
can contact him by email:

davidm at ozemail.com.au

and I'm sure he's be happy to point you to a copy. I grew up in England at
a time when birdwatchers used the term and considered it obvious that the
origin was the "General Impression and Size" acronym that was used during
the war - actually by the Royal Observer Corps than the RAF (my
grandfather was in former, and used the term a lot; I was in the latter,
and never heard it once in a strictly air force context). However, the
picture is fascinatingly more complicated than that, and my own hunch as a
linguist is that the term had more than one origin, and several "false
etymologies" have merged along the way to produce something close to an
"urban myth." Read David's piece!

James West