Is there a reason for the "9" in teen slang "code 9"

Various websites that monitor teen slang have taken notice of a mostly text-based form of communication via the number nine, where code 9 or CD9 means, essentially, that parents or authority figures are watching the conversation.

A few non-reputable examples of the term defined can be found here:

• https://www.kxnet.com/news/cd9-121-53x-as-a-parent-you-need-to-know-what-these-mean/

• https://www.cyberdefinitions.com/definitions/CD9.html

Urban Dictionary boils the definition down:

cd9

parents are around!

if your parent are around and you dont want them to see or know what you are texting, or putting on a website like fb, you send cd9 to alert them not to say or put anything that could get you into trouble!

I'm simply wondering why the 9? How did "code 9" come to mean that parents or authority figures are watching?

So this isn't a sure thing, but I believe Code 9 comes from network error codes.

On the Urban Dictionary page for Code 9, the earliest entry was June 9, 2004:

There is a new online code out, kinda like lol or brb. This one is a little different though. You all know how it feels when you're talking to someone online, and your Mom is standing right behind you, reading every word that is on the screen. Then of course, the other person swears or talks about how much you luv your crush or something and your Mom reads it and tells you to get offline that instant, and not talk to that person anymore.

Well, what can we do about that? To solve this problem, now we have started the "Code 9" system.

In code 9, u simply press "9" when your parent or sibling is watching over your shoulder as you type.

That way, the other person will know what you are talkin about, and begin a conversation about homework or something.

When your Mom or dad leaves, press "99" to let the other person know that they r gone, so u can have a normal conversation again Tom:hey you go out with your girl tonight

John: 9

I did a Google search for Code 9, limiting results from January to July of 2004. The first hit was someone asking about an error code 9 from the ping command. This got me thinking: A lot of Internet slang comes from early hacker culture. Maybe this was the case too. So I did some more research, and found that IETF RTF 1122, Section 3.2.2.1 describes a number of codes related to Destination Unreachable (meaning that you are unable to connect with a host):

3.2.2.1  Destination Unreachable: RFC-792

            The following additional codes are hereby defined:

                    6 = destination network unknown

                    7 = destination host unknown

                    8 = source host isolated

                --> 9 = communication with destination network
                            administratively prohibited

                   10 = communication with destination host
                            administratively prohibited

                   11 = network unreachable for type of service

                   12 = host unreachable for type of service

Code 9 is communication with destination network, administratively prohibited, which seems to fit nicely with your parent blocking any questionable conversation.

Again, this is certainly not definitive, but conjecture.

As a code 9 user from back in the day (our variant was 999), I always assumed it was derived from "911" to indicate "emergency." Of course, a parent reading over someone's shoulder may realize that 911 would be some kind of alarm, so shortening it to "9" helps to obfuscate the meaning. This is anecdotal I know, but something so organically viral like this may not have an "official" historical origin.