My use of kickback goes back to late 90s / early 00s.

A kickback was more "formalized" than just hanging out, but smaller than a party. Same general premise of there being drinks and food at someone's house at a certain time and date, but the expectation is that it would be much more chill. Maybe 6-10 people or so, hang out, drink some beers, maybe smoke a bowl, that kind of thing.