Rhetorical greeting, similar to "What's happening?" or "How do you do?"
The second "t" in "textin" is often dropped, (along with the passe "g"), so that it
may be smoothly pronounced as "texin".
It's especially widespread in hip towns (
ex. Madison, WI and Austin, TX), because it can almost be construed as a little funny in a mutually deprecating way.
Also
popular with the youth, because texting excites them.
Grown Man #1: Oh geez,
Lindsey, I didn't see you there behind the door. Who ya textin?
Grown
Woman: Shut up, Paul, I am hiding. Back away and act normal.
Grown Man #2: Paul! Who ya textin, big guy! Say, have you seen a grown
woman around here? I need to test her for something.