The "Five Eyes" term has been in use since 2006. Strangely enough the original Wikipedia document archived on the Wayback Machine seems to be unavailable, however we do know it pointed to "USAUK_Community" whose relationship was extended to include other allies which is now known as the "Five Eyes".

Before 2007 the amount of searches for "FVEY" was near zero. The relationship between these five nations wasn't named publicly until 2006 or so, before that the relationship was there - just under another name.

However there are documents dated to 2001 available on mors.org which have FVEY references: http://www.mors.org/UserFiles/file/82nd-Symposium/Form%20712...

These are public so you can assume internal naming was used well before then.

FVEY is the accepted shorthand for AUSCANNZUKUS classification, and has been used since the 50's when UKUSA was expanded.