It was first used in Nov or Dec of 1999 right here.
can't find an earlier mention of SERP - there's only a couple of WebmasterWorld threads from 1999 indexed by google - including this one. Wow. domain name snagged. - WebmasterWorld Community Center forum: http://www.webmasterworld.com/forum19/72.htm [webmasterworld.com] haha!
Can you lay your hands on a listing of the WebmasterWorld subforums that existed in late 1999? Back then, the rate of posting was low enough that a brute-force search should turn it up. Unless some posts have physically disappeared. I notice that some of the oldest subforums appear to have started in 1969 [sic], which is an obvious danger sign.
September 2000. Do we have to ditch the "late 1999"?
The paid_inclusion subforum goes back to February 2000. (And seems to have closed up shop in 2006, so it's one of the places I would not have thought to look. My goodness, what a lot of attention AltaVista got back then :))
In [webmasterworld.com...] (linked from one of the threads named above) in June 2001, Brett says explicitly
When I coined Serp
Note how capitalization at this early date is still variable. In the September 2000 thread-- which must now count as our terminus ante quem-- it's "serp", lower case.