I’d say 2014, if not 2015. The term big 6 only got created around that time when Sky realised they needed a way to keep including United in conversations at the start of their decline during the Moyes and Van Gaal eras. Before that point the media only really spoke about the top 4 club and we were routinely left out of that. We became a big 6 level club in like 2009, but the media actively ignored the progress the club had made at the time for a while.
I vividly remember going on Facebook and twitter at the time and a pretty common opinion was people wondering where this ‘big 6’ term came from and I’m sure a few others remember that as well.
There have been changing terms for years. Even the top/sky 4 era also mentioned the "ever presents"
I like that one as it excludes Newcastle, Villa and City.



