When you're talking to someone (mostly takes place in online chat conversations) and all they do is respond with the very annoying, aggravating word "Okay." Hence "shortlining." It usually happens when you're talking about something important too.
Joe: Hey how was your day?
Joe: Well, today I went cruising with that girl I like ;)
Joe: ... Well after I got to know her I finally confessed my feelings to her and she said she felt the same way!
Joe: ... You know what? Quit shortlining me and fuck you dude if you're not interested.