"Tina" is short for "Christina" which is a riff off of "Crystal" as in "Crystal Meth".

"Tina" was used as a code word for crystal meth so people could talk about it in private while in public.

Example:

Person 1 - "Hey, let's go to this party!"
Person 2 - "Is Tina going to be there?"
Person 1 - "I hear she's gonan stop by."

That means that theres going to be crystal meth at the party.

So it's just a code word that worked it's way into our vernacular.

:-)

Rocky