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
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 126.96.36.199 describes a number of codes related to
Destination Unreachable (meaning that you are unable to connect with a host):
188.8.131.52 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
10 = communication with destination host
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.