>>718335341
I did a quick look, and it seems the term first pops up in the 1950's but I can't find any connection to what or why. In the 60's and 70's it was connected to disco dancing and aerobics wear, and then as you can see from the attached image, it really took off big from the 1980's onwards.

As for the tactical use, they're an all-weather garment and you can throw whatever technobabble you want into them, like water reclamation, heating or cooling systems, body armor, maintaining pressure (high alt flight suits) etc. plus they're sexy.