The Avenger's have had many spin-offs but this was a nickname given to the main team after the heavyweight founders (Giant Man, Wasp, Iron Man, Thor) left the book and charcters without a book took over, marking a shift in the character of the team and the beginning of a trend towards frequent line-up changes and less-powerful members. A move away from the JLA model.
I was hoping someone like Julian Perez or Kurt Busiek would have the answer to this.
It's possible the name was not used until #150-151. It definitely appears there.
If it was ever used back in the actual CKQ days, I think it must have only been in a lettercol...
kdb