The earliest I’ve personally seen are from 2005.
Source: Teen culture
Year: At least 1996
OK to Use: Only if you want to sound like a dude
Chillax is a combination of chill and relax. Although top Internet search results say chillax originated with the 2003 film Final Destination 2, the word actually shows up on the Internet in personal ads [<<not-working link] going as far back as January 1996…
Also mentioned at that link: The American Spectator, Volume 36, Saturday Evening Club, 2003, [unknown page number]: “He told me I was overreacting and to ‘chillax.’”