In various toolkits and program’s names there is an ng prefix or postfix. For example: “Airckrack-ng”, “Recon-ng”, peass-ng” and so on.
I’ve tried looking up the abbreviation and reading about those tools but couldn’t find any explanation.
It's not exclusive to hacking toolkits, and stands for "Next Generation". You can verify this by going to https://www.aircrack-ng.org/ , and hovering over the "ng" in the description, where they have helpfully used an abbr tag.