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.