Often used in the imperative, the colloquial verb chillax means to calm down and relax.

It is a blend of the verb chill (meaning to calm down and relax) and of the verb relax.

However, the earliest occurrence of chillax that I have found was explained as a blend of the verb chill, meaning to calm down and relax, and of the verb max, meaning to lounge, i.e., to lie, sit or stand in a relaxed or lazy way—cf., below, Posse up for this slammin’ list, the glossary appended to Slang—it’s a way of life.

The earliest occurrences of the verb chillax that I have found are as follows, in chronological order:

1-: From Slang—it’s a way of life, by Danielle Whitworth, News-Sentinel staff writer, published in The Knoxville News-Sentinel (Knoxville, Tennessee) of 14^th August 1992:

Yesterday I was chillaxin’ at the crib, having a blend session and getting the latest scoop from my dirty dog. He told me a story that was slammin’. So I told him that he was still my boy and gave him some dap for a good try. We grabbed our brims and a couple of homies and we were audi. Headed straight to the mall to get some cooleyhigh gear to bang out in.
Confused? It’s only the latest slang.
[…]
[…] Some words are understood by teens coast to coast, thanks to famous rappers and what is seen on cable television stations BET and MTV.
[…]

Posse up for this slammin’ list

Trippin’ or Buggin’—Acting strange.
“G”—Way of addressing someone, as in “What’s up, G?”
Bang Out—To wear nice clothes.
Chillin’—Relaxing.
Maxin’—Lounging.
Chillaxin’—Chillin’ and maxin’ at the same time.
Play me to the left—Brush off (used mainly by guys when being rejected by a girl).
Slammin’—Excellent.
Straight—That’s Fine.
Dope—Outrageous.
Blend Session—Haircut.
Cooleyhigh—Casual.
Foul—Wrong.
Jet—Leaving.
Audi—Leaving.
Peace Out—Goodbye.
Gank—Take.
Carry—Move on. (Usually stands alone.)
Step—Approach. (Don’t get this confused with Step Off, meaning go away.)
Cuz—Way of addressing someone.
Down wit—OK with that.
Hard—Cruel.
Dis—Disrespect.
Sweatin’—Pushy. (Can be used in either a good sense or bad.)
Mack/Mack Daddy—Playboy.
Macguyver—Impossible.
Frontin’—Being Fake.
Bone—Dollar.
Whupped—Ugly.
Ducats—Money.
Posse Up—Get Together.
Scoop—Gossip.
Grub—To eat.
Dap—Credit for doing a good job.
Brim—Hat.
OTB (Out There Bad)—Crazy or spaced out.
Whack—not good.
Gear—Clothes.
Homey—Friend.
Dirty Dog—Really good friend.

2-: From Devil worshipping mom beaters, posted on 7^th December 1994 in the Usenet group bit.listserv.cinema-l:

Wesam Esmail
you guys don’t even deserve this, but i think someone should tell you that you’re a bunch of guys that know as much about criticizing a movie as a pig knows aobut taking a bath (pig is a filthy animal, right?)
Fern Cervantes
Chillax my friend. I agree with most of your sentiments about Tarantino and his use of violence as comedy. However, this juvenile form of attacking others who don’t share your opinion isn’t going to help you win anyone over to your side. Just take it easy and “be cool.” You’ll find that everyone here is quite friendly and gets along just fine despite any differences of opinion. I’m not sure what exactly set you off but everyone’s opinion is just as valid as yours, as long as they honestly express what they feel. So kick back and just enjoy the list for what it is: a forum for expressing different opinions on cinema.

3-: From Hello from a 1^st Timer, posted on 8^th September 1998 in the Usenet group uk.music.rave:

Gip
yo wut up, home slice! welcome to our crib. we’re usually just kickin back and chillaxin, ya know wut i’m sayin. but yo, check this, there be some weird ass peeps on this ng. but it’s all good, ya know wut i’m sayin. every now and then ya gots peeps who think they gonna put their beat down but they just talkin some smack, yo!

4-: From Jiggy jive, a guide to “teenspeak”, published in The Tampa Tribune (Tampa, Florida) of 31^st October 1998:

Chillin’ or chill’axin: relaxing or hanging out together.

5-: From recommendations to students, published in The Monitor (McAllen, Texas) of 1^st April 2000:

Just chillax
Before doing anything unhealthy to your body in your quest to combat the anxiety, reach from within and follow these quick and easy tips to stomp the stress as prescribed by Joseph H. McCoy, licensed psychologist in Mission.
1. Take a break and do something different. When you are studying for that AP exam, have a snack or watch some television.
2. Do something physical—particularly aerobic exercise. This will release some of your anxiety on a physical level.
3. Look at a situation from a different perspective. Bring yourself out of your situation, and look at it as if a friend were explaining the situation to you. When you look at it this way—is your situation really that bad?
4. Try to find humor in the situation. Sometimes situations are humorous after you process what happened.
5. Talk to a friend or keep a journal. Don’t internalize—let it out, whether by talking or writing about it.

6-: From Learnin’ all ’bout talkin’ smack, by Chris Ardis, published in The Monitor (McAllen, Texas) of 5^th June 2000:

McAllen—As we near the one-year anniversary of the first edition of Teen Talk Dictionary, what more appropriate way to celebrate than with Teen Talk II. You won’t find these words in any dictionaries in your local library, but you will hear them coming out of the mouths of most any teen-ager in your life.
Stoked: To be pleased. “I was totally stoked when I heard that Lupita is dating John.”
Baked: Same as “stoned” or “high.” “That guy was so baked he thought he saw aliens in his fridge.”
Later days: Get lost. “You think it’s my fault that Jose broke up with you? Later days!”
Smack (I love this one!): Talkin’ trash. “You don’t know what you’re talking about. You’re just talkin’ smack!”
Chillax: Combination of “chill” and “relax.” “Chillax, mom, I’m gonna pass that physics test tomorrow.”
Peeps: Short for “people.” “Hey, check out all the peeps in line for that movie.”
Trippin’: Messin’ around, lying. “When Mike told me that his sister was going to be on Who Wants to be a Millionaire, I told him to stop trippin’.”
Bust it: (another cute one): Making out. “Why do those two always bust it right in the middle of the cafeteria?”
Lates: Talk to you later. “Gotta get to class. Lates!”
Like: This one is ever-popular and one that makes me absolutely crazy when talking to teen-agers. The conversation usually goes something like this: “We were like, going like to the mall and, like, these guys in this, like, pimp car (remember from the first edition of Teen Talk that “pimp” means really cool) pulled up and like, started like talking smack to us. We were, like, ‘Are these guys trippin’ or what?’ And, like, they wouldn’t stop following us. We finally all, like yelled out the car, ‘Later days’ and like, just took off. It was, like, crazy.”
Sound familiar? If it does, you probably have a teen-ager living in your house or talking to you on a regular basis.
I was like, a little stoked when I learned that like, several of the words from the first edition of Teen Talk are like, already old.
I feel much better now because I, like, know the new words. Now I can, like, chillax and know that I am, like, with it. Lates! (Note: Many thanks to Danielle Heath for helping me stay cool!).

7-: From The 411, published in the Dayton Daily News (Dayton, Ohio) of 6^th July 2000—“The 411 is a weekly column of what’s hot and what’s not, compiled by youth correspondents for Young People’s Press, the North American youth news service”:

Say it cool: ‘Chillax’
This is the word to use if your friend’s spazzing and you want to get them to chill out and relax. So it’s like: “Chillax, buddy.”