When a person is in a state, where he can't or won't talk to other people around him/her. Can also be called headphones mode.
Autistic mode can be achieved easily by putting headphones on and play some good music. Noise is an excellent insulator against talking.
This state can be also achieved without phones by concentrating hard on to something or by dreaming (class or any other boring occasion).
Makes people hard to talk to because they don't react even if you call their name.
Comes from anime series Ghost In a Shell episode 22 where Ishikawa and Batou are tracing Chief Aramaki, but cannot contact him.
Ishikawa: That's strange. The Chief's running on autistic mode
Mom: So what have you been doing lately? This room is a mess! When are you going to clean it? Where is that nice girl we saw the last time we visited here?
Me (to myself): Fuck it, I'm going autistic mode (puts his headphones on and plays some Infected Mushroom)
Friend: And then she said to go left but i went right, so i kinda felt stupid but then she instructed me......
Me(to myself): Man, this math question is way too hard. Should have listened to the teacher, when he taught this. What was I thinking then? Oh yeah, Laura. And what would she look sitting on my face. Hmmm... Nice.
Friend: Calvin! Caaalvin!
Friend: Did you or did you not remember to return that book? Damn, are you always on autistic mode?
Me: Sorry. Yeah, book's returned.