The story
From seven-level + screwdriver, so called because Air Force "seven-level" personnel (advanced techs/higher ranks) typically carried this kind of screwdriver. It was rumored to be the ONLY tool these advanced techs ever needed, or were allowed to carry. Advanced techs only "do" small trim-pot tweaking. Less-experienced techs handle the heavier tools and tasks.
Etymology from Wiktionary, CC BY-SA 4.0.